I'm with you guys. Thought it was common knowledge.
But I heard that onlooker was adamant, even said something like, "You don't know you can't swap more than once in a point? That's Tennis 101!!!"
It sounded like the guy was either a league captain or league player not able to mind his own business.
Anyway, as much as I'd love to pass this thread along, rather is there a rule in an official tennis rulebook somewhere that will put a cap in it?