It unfortunately isn't worded that it only affects new usage. If you needed to check once before your initial use that would be shady but unquestionably legal as the terms of the contract are clear when you are entering into the agreement.
This clause allows them to arbitrarily change the contract with you at will, with no notice. That _shouldn't_ be enforceable but AFAIK that kind of contract has never been tested. It is _likely_ unenforceable though.
Why shouldn't that be enforceable? "Permission to use this software may be withdrawn at any time" seems like it would be an enforceable clause; why would this be less so?
This clause allows them to arbitrarily change the contract with you at will, with no notice. That _shouldn't_ be enforceable but AFAIK that kind of contract has never been tested. It is _likely_ unenforceable though.