Well, multiple boot machine is something very nice but we must be careful with all we do when we're installing the OSs

There is a long time I don't install Fedora, but ubuntu can specify the boot partition in the installation interface it uses now. I think Fedora also give us a lot of facilities.

I don't know how you did your installation, but I use to create few partitions in the HDD before install anything. I use a / partition where I install each OS (about 20 GB, perhaps 10 GB will be enough for that). A /home partition where I store all my information, the size will be as big as our HDD let us create it. And a swap partition about the same size of my RAM for hibernate and suspend the PC correctly, very useful for portable PCs.

This way let us to have more than  linux SO installed and also change it or upgrade it if necessary without loosing our data.

On the other hand, why you upgraded Ubuntu? The LTS release are very comfortable for work, you can get a very stable and updated system for years. Only if you have troubles with your hardware or if the LTS don't allows you to explode a feature as you need or want, will be a good idea upgrade it. But if it works, don't break one of the golden rules of the Engineering: if it works, don't fix it.

Finally because the GRUB, exist the Supergrubdisk which allows you to fix some common troubles with the GRUB and also you can change manually the grub.conf file. There are a lot of tutorials in blogs, mainly in the Ubuntu forums, you can find for do it. Once I had to do it.

Good luck