Not sure if it's something weird with my system, but running that exact command I get

Error: Unknown repo: 'updates-testing-modular'

Dropping the reference to updates-testing-modular gets me a couple of removed packages (obviously the kernel modules are not an issue, but I don't know what the others are for) along with some downgrades.

sudo dnf --releasever=34 --setopt=module_platform_id=platform:f34 --enablerepo=updates-testing --allowerasing distro-sync
<snip>
Removing:
 kernel                                                         x86_64                     5.10.14-200.fc33                                               @updates-testing                                                        0  
 kernel-core                                                    x86_64                     5.10.14-200.fc33                                               @updates-testing                                                       74 M
 kernel-debug-devel                                             x86_64                     5.10.14-200.fc33                                               @updates-testing                                                       53 M
 kernel-devel                                                   x86_64                     5.10.14-200.fc33                                               @updates-testing                                                       53 M
 kernel-modules                                                 x86_64                     5.10.14-200.fc33                                               @updates-testing                                                       30 M
 kernel-modules-extra                                           x86_64                     5.10.14-200.fc33                                               @updates-testing                                                      1.9 M
Removing dependent packages:
 NetworkManager-ovs                                             x86_64                     1:1.26.6-1.fc33                                                @updates-testing                                                       76 k
 axis                                                           noarch                     1.4-34.fc27                                                    @System                                                                13 M
 empathy                                                        x86_64                     1:3.12.14-7.fc29                                               @fedora                                                                15 M
 kmod-nvidia-5.10.14-200.fc33.x86_64                            x86_64                     3:460.39-1.fc33                                                @@commandline                                                          43 M
 kmod-xpad-5.10.14-200.fc33.x86_64                              x86_64                     4.14-1.20170915gitbbc8608.fc33                                 @@commandline                                                          58 k
 openvswitch                                                    x86_64                     2.14.0-2.fc33                                                  @updates-testing                                                      5.3 M
 rdma-core                                                      i686                       33.0-2.fc33                                                    @updates-testing                                                      114 k
Downgrading:
 abrt-java-connector                                            x86_64                     1.2.0-2.fc34                                                   fedora                                                                 46 k
 balena-etcher-electron                                         x86_64                     1.5.99-1                                                       etcher-rpm                                                             60 M
 blender                                                        x86_64                     1:2.91.2-4.fc34                                                fedora                                                                 43 M
 conmon                                                         x86_64                     2:2.0.22-0.9.dev.git2fbeb9f.fc34                               fedora                                                                 48 k
 containernetworking-plugins                                    x86_64                     0.9.1-1.1.gitfa48f75.fc34                                      fedora                                                                8.8 M
 containers-common                                              x86_64                     1:1.2.1-28.dev.git1b813f8.fc34                                 fedora                                                                 79 k
 firefox                                                        x86_64                     85.0-11.fc34                                                   fedora                                                                102 M
 fuse-overlayfs                                                 x86_64                     1.4+dev-2.dev.gitee8ce2e.fc34                                  fedora                                                                 76 k
 gecode                                                         x86_64                     6.2.0-5.fc34                                                   fedora                                                                3.1 M
 gnome-abrt                                                     x86_64                     1.3.6-2.fc34                                                   fedora                                                                140 k
 httpd                                                          x86_64                     2.4.46-8.fc34                                                  fedora                                                                1.4 M
 httpd-filesystem                                               noarch                     2.4.46-8.fc34                                                  fedora                                                                 13 k
 httpd-tools                                                    x86_64                     2.4.46-8.fc34                                                  fedora                                                                 82 k
 libuv                                                          x86_64                     1:1.40.0-2.fc34                                                fedora                                                                152 k
 libuv-devel                                                    x86_64                     1:1.40.0-2.fc34                                                fedora                                                                 29 k
 libuv-static                                                   x86_64                     1:1.40.0-2.fc34                                                fedora                                                                116 k
 mod_lua                                                        x86_64                     2.4.46-8.fc34                                                  fedora                                                                 62 k
 podman                                                         x86_64                     2:3.0.0-0.204.dev.gita086f60.fc34                              fedora                                                                 11 M
 podman-docker                                                  noarch                     2:3.0.0-0.204.dev.gita086f60.fc34                              fedora                                                                184 k
 podman-plugins                                                 x86_64                     2:3.0.0-0.204.dev.gita086f60.fc34                              fedora                                                                1.3 M
 podman-remote                                                  x86_64                     2:3.0.0-0.204.dev.gita086f60.fc34                              fedora                                                                8.6 M
 reportd                                                        x86_64                     0.7.4-4.fc34                                                   fedora                                                                 50 k
 strongswan                                                     x86_64                     5.9.0-3.fc34                                                   fedora                                                                1.5 M
 strongswan-charon-nm                                           x86_64                     5.9.0-3.fc34                                                   fedora                                                                 34 k

On 2/20/21 3:49 AM, Miroslav Suchý wrote:
Do you want to make Fedora 34 better? Please spend 1 minute of your time and try to run:

  # Run this only if you use default Fedora modules
  # next time you run any DNF command default modules will be enabled again
  sudo dnf module reset '*'

  sudo dnf --releasever=34 --setopt=module_platform_id=platform:f34 \
    --enablerepo=updates-testing --enablerepo=updates-testing-modular \
    distro-sync

This command does not replace `dnf system-upgrade`, but it will reveal potential problems. You may also run `dnf upgrade` before running this command.

If you have have rdma-core.i686 installed you have to pass `--allowerasing`.
https://bugzilla.redhat.com/show_bug.cgi?id=1919864


If you get this prompt:

  ...
  Total download size: XXX M
  Is this ok [y/N]:

you can answer N and nothing happens, no need to test the actual upgrade.

But very likely you get some dependency problem now. In that case, please report it against the appropriate package. Or against fedora-obsolete-packages if that package should be removed in Fedora 34. Please check existing reports against
fedora-obsolete-packages first:
  https://red.ht/2kuBDPu
and also there is already bunch of "Fails to install" reports:
  https://bugzilla.redhat.com/show_bug.cgi?id=F34FailsToInstall

Thank you

P.S. sent from workstation successfully upgraded to F34 :)