Nothing much really... A set of IO ports are missing on the m8 and the register names are slightly different so you have to look it up on the datasheets.
It has less memory too... As much as I am inexperienced with uCs in general, I have been able to successfully port several programs of mine from 1 AVR to another without a problem.
If you tell me what your program does (i.e what all it uses--interrupts, timers, ADC etc) then I might be able to tell you what you should look out for while switching uCs.
If I can't help, just head on to avrfreaks.net.
