have a look here
look at me!! youll find all the info you need, as long as you aint got a TA-082 mobo, you can upgrade to 2.71 then
down to 1.5, or emulate 1.5 from 2.71. but downgradings your best option, just read everything
twice or you could end up with a brick!!
read up on custom firmware aswell, it will save you from most possible bricking situations

wtf- just realised the first post was over 2months ago?!?