I would think that switching to a racket considerably lighter or heavier part way through a game can't be too good for the body.
Purely because the speed at which you swing is going to be vastly different and whilst going lighter is easier on the overall muscles and manouvrability, it could cause strains in other ways. Just my thought.
I can see why having a lighter racket can help with fatigue, but going too much lighter could have some negative effects on the shoulder/elbow etc.
I do also see the point when one is playing an opponent that is perhaps a real beginner or just not that good and using a racket that hits a heavier ball is really necessary, so having a play about with a different racket can just keep it fun and a bit interesting, to make more of a game of it, even though the outcome is inevitable. I've actually got an oversize Prince racket that is a about 15g lighter than my 300 that I have thought might be fun to have a ping about with sometime.
"The older I get....the better I was"