I might be wrong but would the trick maybe be to break it down bit by bit, so for instance break off your existing file upload stuff and replace it with CarrierWave or something different. I don't know what your codebase is like but doing it that way should in theory make it more copable to slowly tick it up to rails 3+
Gotcha, there's also some information on migrating to paperclip[1] which might suit and I'm sure someone will have done similar for CarrierWave. Of course none of that helps with being short staffed.