I change rackets (I carry two). I don't do it all that frequently during a match, and I usually do it if one overgrip feels slippery. I will change more on hot days or in singles.
I remember one match where I just couldn't seem to get any directional control on my serve. I changed my racket at love 30. I held and served well after that. Go figure.
Cindy -- who always puts the extra racket back in her bag rather than leave it lying around because she is absent-minded enough to go home without it
-- Random Error Generator, Version 4.0
-- Master Moonballer