The only thing I found was this: https://professionals.collegeboard.org/guidance/applications/ethics
Basically, they say it is acceptable only when you are waiting to hear back from other schools (as you are), and I would have to agree. If you do decide to cancel your acceptance and accept a second offer, I would just let the first school know ASAP so that someone else has a chance to take your spot. Honestly, these different programs should try to figure out a universal timeline so that this is not even an issue.
Best of luck.