My understanding is that Excel will just convert  them (perhaps earlier
versions like 97 or 2000 though).

Could well be wrong though.

I ran into the same problems with my engineering estimate I
developed for my company which contains 16,300 lines of code. I
pretty much manually transferred the code to the new version. There
were some areas that had to be rewritten, but over all not too bad.

The userforms weren't too bad either, instead of showing a "dialog"
sheet, you just do a search/replace to show the new userforms...