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There are some paid users of Twitter, called Twitter Blue [1].

[1] https://help.twitter.com/en/using-twitter/twitter-blue

Sure, but if I don't know what they're changing it's hard to know if I want to cancel or not.

Is this the case in the U.S.? Got any handy references to specific laws in this regard?

This is basic contract law but you'll likely need to review the specific contract in question to see if they have any clauses you've agreed to about cancellation.

But if you have an agreement that one side unilaterally changes and the other side doesn't agree you probably don't have a contract anymore.

Can you demand they return all the personal data they've taken under the old lisence? Because you pay them in data.