I am not a captain, but I have played mainly doubles for two tennis teams. I have to say a good partnership is the key to winning in doubles. I played on one team where I had a new partner every game and it didn't work very well. We never got into a rythm. The other team I played, I found a partner that worked well, we lost a few of our first matches, but we began to get on a roll towards the end of the season because we played together. We'd pick-apart teams that just showed up that day because we knew how to play together, our tendences, and how best to position ourselves.
I'd be hesitant to break up a an already established doubles pair if they perform pretty well.
My advice would be to keep the doubles pairs that perform pretty well and consider breaking up the doubles pairs that don't. Combine the new players and the weaker pre-existing doubles based on your knowlege of thier games, strengths, and weakness. Give the players the options to switch if the pairing doesn't work out or stick with thier partner for a if it works out.