are sponsors legally bound by their official rules for contestsWhat is required is they decided to change or alter those rules?

Question

are sponsors legally bound by their official rules for contests

What is required is they decided to change or alter those rules?

Answer

I think a threshold question is whether the so-called contest is legal, or is instead really an illegal lottery or raffle. See Penal Code sections 319 and 320.5, for example (can be found on line or at a county law library). If the contest is legal, the next question is going to be whether the rules (which are probably contractual in nature) expressly or impliedly allow the proposed change. Probably not. If the rules change and the contestants, or some of them, are adversely affected, they would probably have grounds for lawsuits against the sponsor. Would they sue? Probably not, since their individual losses or prospective losses would be small. Might there be a class action? Possibly. To give you a better answer, we'd need to know a lot more about the nature of the contest, identity of the sponsor, and particulars of the rules and the proposed change.