I have never attempted using a compression fitting over a previously soldered pipe, but my guess is that if you heat the pipe then wipe the residue solder well, as long as the compression ring will slide on the pipe, it should be OK. If this is incorrect, I'm sure some of the pros will correct this. Redoing any old joint, the key is getting the old solder off it pipe, (as described above) and cleaning the solder out of the fitting. The real problem is getting the old fitting cleaned out well enough to get the joint reassembled. I think most of us find that trying to reuse a fitting is not worth the small cost of a new fitting, but it can be done. Other than the cleaning the old solder, treat it like any other joint.