When I did the upgrade from r5a12, I knew it took some tweaking to get things just right, so I made sure to back up everthing I might have touched before upgrading, that way if I had to I could go digging throught the backed up files and find what I changed--even if only for reference.
Here are some things that are useful to backup manually:

/etc (this gets your XF86config-4, lirc stuff, modules stuff, etc.)
/user/share/mythtv/themes (if you've added or customized themes in any way--the front end wont start if it's missing customized themes)
/home/mythtv (i know the backup function backs up your home, but I grabbed it anyway, not knowing ahead of time. It's handy because you can grab files one at a atime if you need to without untarring/zipping).
channel changing script

Here's what I did:


-p preserves ownership, permissions, modification dates, etc.
-r is recursive, ie, backup this directory and all subdirectories & files.
cp --help for more info

When I had to revert back to r5a12 here's what I did
reinstall nvidia drivers (do this first or you'll break X)
copy the themes back


cp prompts you for confirmation (i believe cp is aliased to cp -i), so --reply=yes gives it a yes across the board

drop your backed up /etc back in place:


if you use a channel changing script for satellite receiver, be sure to put it back:

be sure to set execute permissions for whoever needs to run the script. I just use give everybody execute on the channel changer with chmod ugo+x. Just make sure only root has write!

This only likely to work (especially the /etc part) if you're going back to the same version you were on when you backed up.

Also for some reason after the restore, everything in /etc was owned by mythtv:mythtv. Not sure why. either I typed something wrong or I messed up some options on the cp. At any rate cp --help and man cp is your friend.

Anyway that resulted in a 20min restore on the first try.

Now if only for better luck with r5a16 this weekend...