Bert,
Sorry I didn’t truly before this, but I don’t see any difference between renaming a file a renaming a folder.
You say you "rename registrations all the time". If so, you know the procedure is:
1. Locate the file.
2. Press the Name (1 down) button.
3. Select the file you want renamed.
4. Press OK (7 down button)
5. Enter or edit the name.
6. Press the OK button (8 up).
Renaming a folder is the exact same procedure, except where it says "file", substitute "folder".
You can have up to 500 files in a folder on modern Yamaha keyboards (250 on older ones). There is no limit to the number of folders you can nest within other folders, except that each folder uses up one filename space. No filename can be longer than 50 characters including its extensions.
Fred