Yes you can flash it back to your 6100A. As the instructions noted wit hthe 6500A you should backup your old firmware before flashing to 6500A anyway.
For 6100A firmware (if you havent backed up your original) I have my old one from a year back on my hadrddrive here.
Private message me and we can deal with it that way.
Note: easiest way to flash is using binflash
if you want to try other (better) 6500A firmwares, check here: NEC 6500A firmwares
I suggest the Liggy 2.24 (if yours is v2.xx) or 4.24 if your drive is 4.xx