Can a school change their admission decision before April 15th?
I am very experienced with admissions in the U.S., in particular in math programs at my R1 university: unless you grossly falsified something in your application, it would be very unusual, and substantial professional misconduct, and would not be even contemplated as a dignified option, for a grad program to recant on an offer of admission and funding.

The only to-me-conceivable scenario would involve catastrophic budget problems at the university ... since, after all, grad student funding is a tiny bit of the whole.

So, at least approximately, I'd recommend thinking of the offer of grad admission and funding as a far more reliable contract than most others you'll encounter in life. (Car repair estimates? Real estate appraisal? Promises of eternal friendship :)

So, quite seriously, you should not worry about it (if the offer is from a reputable, long-established university (e.g.) in the U.S. that is not filing for bankruptcy etc.