When does Compressor/FCP actually compress?

Hey all...

So here is a question that has been bugging me....

When does Compressor or FCP actually compress?

For example.....I want to change the framerate of some DVCPROHD footage. So I bring it into comrpessor, choose the same framesize and codec settings, but change the framerate, and "compress".

Will I have just compressed my footage for a second time in DVCPROHD (the first being when it was shot?) Or does it recognize that it doesn't actually need to be compressed and "leave it alone"?

Thanks everyone!

g