Well, a couple things.
First, my files are on a network, and when I changed the extension to .m4a, it actually renamed the files to have that extension, even though they were .mp3 files (this was what was going on earlier - they WERE mp3 files - they had just gotten renamed)
Second, when I changed the path information, it MOVED the files. I had to answer the "are you sure" question 600+ times. Ouch!
I think I got it all figured out and got the files renamed and moved, and all back to where they should be (I think). Then I disconnected my connection to the network, and did it again. MC said it couldn't update the tags, but did anyway, then once I pointed the library to the right location, with the right extensions, it 'connected' the files. It didn't associate them all, but i'm pretty sure that's because of QuickTime's inability to read long file names. I'm sure if you did this with another file type, it 'should' work.
**Note** I had to 'disconnect' my network drive to make MC unable to actually write the changes (which moved and renamed the actual files. I don't know how to do this if they are all on a local machine. I guess you would just have to move the original files to another directory outside og MC to 'break' the connection, then find/replace to point to the right place/file.
Hope it helps.
Be sure to backup before starting. I actually created a new library, which was cloned from my original, that way my original was unchanged (good since it got jacked up)