Thanks again for jumping right in there Tapeworm!
I found the Mp3tag but is is v. 2.51 and didn’t addess anything resembling ID3v2.3 ISO-8859-1 except maybe the v. #.
Under Tools/Options/Tags/Mpeg set Write to ID3v2.3 ISO-8859-1.
Once you have this default setting changed, you can open up a folder at a time (or the whole library) and just hit the SAVE icon (looks like a floppy disk) and it will save all in the new tag format.
While you’re at it though, you may want to look at the files individually to see if you need (or want) to make any other changes. Delete anything in the Comments field. Your player can’t read it and stuff in there can cause issues.
It’s best to make these changes while the files reside on your computer. If you have copies already there, go ahead and delete the ones on the player. If not, transfer them back from the player to the computer, make the changes and then copy them back.
MP3Tag won’t change your folder/file hierarchy.