Yes, I fiddled with a software called InfinityBox but you have to buy a Dongle, but it's costly, at the time I bought my Nokia 603 (which was like 4 weeks ago) the dongle costs almost as much as my Nokia.
That software can downgrade, and when using it you wipe all the info of the phone, but you need to backup a lot of things such as PP, Signatures, CMLA Keys, the latter are necessary also but you need the fully featured software to do the downgrade.
So what I did was sell the phone which had Feature Pack 1 FW, then bought another 603 new in box with the original FW (Belle) and stuck with that version.