Accepting a school off of the waitlist while SIR'd to another school.

So if I’m currently on the waitlist for one school but I already SIR’d to another school, do I need to cancel my SIR before accepting the offer? I just want to make sure I accept it within the time frame, and I’m not sure how quick the school would be able to cancel my SIR.

You would accept the waitlist offer first and then withdraw/cancel your SIR from the current school.