Not against the rules to switch racquets. I once played a guy in a tournament that used 4 different racquets. I asked him about it and he told me they were all demos and he was in the process of switching racquets. A tournament is an odd time to demo- but hey whatever- it is his choice. He even gave me his thoughts on the racquets on changeovers-lol
He didn't seem to be too concerned about actually winning the match which I thought was pretty funny.