It is possible to easily manipulate an existing firmware to make it flashable on a different drive. Flashing back is a bit more tricky as you need to modify the software needed for flashing. At one place you need to issue a 3500 command because you have a 3500 firmware, but your drive is still recognized as a 2500 at other places.
I guess I could provide you with a modified version of Binflash that allows you to flash back to 2500. Just drop me your eMail address in a PM.