When changing files I suggest the following:
1) Copy SD card files to a folder on your computer.
2) Format the SD as described here: http://www.benheck.com/amh/
3) Make changes to the SD folder on your computer.
4) When done, copy it back to the SD card all at once.
The reason for this is making multiple changes to SD card can fragment it (yes, as in back when you had to defragment your hard drive) and this can eventually confuse the file system (since it's simple and fast) By making a fresh copy on a blank card, everything is in sequence.
Changing music on RZ won't be nearly as easy (or advisable) as it uses loop points which will "break" if the songs change.