Basically there is no right answer as to when to deal prospects and when not to. For every deal where prospects flamed out and the other team got the 'proven star', there is another deal where the main piece was gone within a year or two and the prospects were key cost-controlled players for the next 6 years.
Personally I think if we have to give up Buchholz then Kelly/Westmoreland should be completely off limits. I don't need to describe what we have in Buchholz, but Kelly is 20yrs old and going to be pitching in AA and Westmoreland has five-tool superstar potential that hasn't been seen since Hanley graced us with his presence.