I do this all the time...change only the pads which are shot, leave the old pads still sealing. It comes out perfectly fine. Indeed, if one pays attention there is a different 'feel' to the keys with new vs. old...but one really has to be paying attention.
I usually repad C-mels with riveted, non-resonator pads since that is what they were outfitted with...just for the sake of history.
I do wonder, sometimes, if one outfitted a C-mel with the most sonically egdy sorta resonator pads available (not that I know if there is such a thing; or if it's just a matter of opinion)...if that would provide a teensy counterbalance to some of the dark stuffiness of the horn.....