Usually this sort of contract only applies for the term of the subscription, and then the company can change whatever they like at the end of that term.

The article even specifies that the grandfathered plan only continues to exist because Microsoft is “allowing” it.

When asked, an Xbox spokesperson did not confirm how long legacy Console subscriptions will remain active for current subscribers.

So I get what you are saying, but in this specific case, there is no legal reason why Microsoft could not force current subscribers to move to one of the new, more expensive plans. They are just being nice to some of their most loyal users, for some reason.