This is pretty ancient history, and a review of all the behavior changes between versions that may or may not affect you is pretty much impractical. That being said, FM is remarkably good (sometimes perhaps "too good") at maintaining backwards compatibility in most cases.
My advice would be to do the conversions, then test thoroughly... then rebuild regardless, since there are about a bazillion ways your old solution could be made better 13 versions later. Seriously, would you want any software, except for maybe old Atari games, to run the same way it was designed to run a quarter-century ago? Good for nostalgia and a few laughs, and of course if you need to retrieve the data, but not much else.
This may help you some, though: FileMaker conversion path for all versions | FileMaker You're going to have to do a couple of intermediate conversions, but I noticed this includes links to trial copies of the intermediate versions you'll need.
older FM db's are often quite simple - so chances are good, that they can be brought to a current version. A year ago, I had to migrate some FM2 files to v13. Not a problem - but took a lot of time. That customer had an old beige G3, it's HD died..
that said, I support Chris' statement to rebuild - although You can keep on working with those converted files (when all goes right), means: No hurry
Thanks for your reply, Chris. I'm pressing on with conversions.
Thanks, Marcus. I appreciate your comments.