Decompile Atmel AVR Hex File to C code

I currently have a hex file that gets programmed into an Atmel AVR ATmega8335 microcontroller that is soldered onto? our PCB. This firmware was last updated about 7 years ago. The original source code is now missing, due to bad engineering documentation habbits,? but I need to update the firmware before next season to upgrade a? product feature. Well, since I don't have the original? C source code then I'm looking for someone who ownes a C Decompiler (like IDA Pro) so you can take my hex file and convert this to? C source code. The original source code was written around 2002 and an IAR C compiler was used, but I'm unsure what version it was compiled in. Please reply if you have the capability to Decompile? a hex file that was written with an IAR compiler for an Atmel ATmega8535 micro, and convert it to C.?