I also start with:
rpm -q --qf "%{NAME}.%{ARCH}\n" -a | sort > rpms.txt
I run it on both old and new to get a lits of rpms
without any specific version numbers. I can then
compare them with the "comm" tool to get lists of
rpms on one system but not on the other.
That doesn't help with all the rpms that have been dropped
or repackages and renamed, but it gets lots of stuff
as a starting point.