These users have agreed to a monthly contract or, if there is no money paid, a contract with no finite end date but with provisions to change terms, essentially terminating and restarting. So the service provider has decided to amend the contract at the end of the current (one month) contract in the first case, or on some date arbitrary date in the second (unpaid). The users are free not to accept the new contracts.

So nobody is just changing a contract mid stream. Use of a service month-to-month implicitly agrees to this: your ability to stop using and paying them on the 1st of the next month is their ability to change the terms on which the service is offered next month.

And btw, everyone on here hates this, but I don't know how else it could work. The idea that if I sell a customer one month of a paid saas on a monthly plan I'm somehow obligated to never change my terms or price forever as long as he or she keeps paying is beyond absurd. If people want stable terms, they need to find software that will sell them annual or multiyear contracts.