When a company changes their terms of service you can go demand they cancel your membership if you don't agree. But since you don't pay for twitter it's sort of moot.
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.
