Hi Wim,
excellent idea for a program, and a clever and simple user interface.
I use a freeware called CKRename which allows easy renaming of multiple files.

Your program is very useful for anyone without a sx6 or sx3 keyboard hard disk.

If I could make one constructive suggestion, the screenshot I saw had space for 10 files. Is it worth having space for 20 files, since in 6000/6500 format it is very easy to fit 20 performance files on a floppy?

However for KN5000 and 3000 owners this is all you should need.

Well done!