Read the EULA
Sadly, yes--read the EULA. You do not have a contract with Microsoft; you have a license. Microsoft is free to change the terms of the license at any time without prior notification or consent. Call me a freetard, but this is one of the great injustices of "intellectual property." It has given near unmitigated power to licensers to revoke rights from licensees, as is the case here. Agreements like this clearly ought to be governed by contract law, and this clearly ought to be a violation of the contract; however, Microsoft claims that you are subscribing to a "service," the terms of which Microsoft may change at will. I understand why companies do this--they care primarily and almost exclusively about their own interests and the interests of their shareholders. What I don't understand is why the legislatures and courts have bought it. The current state of IP law is a monstrous injustice.