Sure, you can update them. You may not have heard about your waitlist status by May 15. Since some people mail their answers back on May 1, it takes several days for the colleges to get and process them. Then they may go to the waitlist, but keep people on it until they know they aren’t going to it any more. It does vary, but you could send the update if you have one. You will, however, have to deposit someplace by May 1, and you will lose that deposit if you accept a waitlist spot elsewhere. And the chances ARE slim of getting off waitlists in general.
Usually waitlist decisions come out in mid-late May, although it’s definitely not unheard of to be removed from the waitlist in June or July. One of my friends last year was taken off the waitlist for UVM (not an Ivy, I know, I know) in mid August. On the other end of the spectrum, MIT made all their waitlist decisions by May 10. It’s hard to know in advance.
If a school is overenrolled (got more “yeses” back than they anticipated from accepted students), they will shut down their waitlist very quickly and hope for some summer melt so they have room in the dorms in the fall! If they are underenrolled, they likely will start going to the waitlist for some students around mid-May. If they are right on, they will keep at least some students on the waitlist for a while in case of “summer melt” where students who said yes on May 1 end up going elsewhere or dropping from their school for other reasons.
So no one can actually answer your question. And for schools not really using their waitlists (which can’t be predicted ahead of time), it won’t matter anyway.