Lawyer up! They unilaterally changed the contract, that probably invalidates that clause in any sane country (so, not USA). They might even have to pay you the cancellation fee, or more.
I agree that this type of change to contract terms is not acceptable, but unfortunately you need a lawyer to fight it.
Similar clauses, some without opt-outs, have proliferated widely throughout the tech industry. Most notably, Windows added them to most of their products shortly before the release of Windows 8. Interestingly, GitHub does not have these clauses yet.
Did their previous contract said they could unilaterally change the contract? :-)
There's all kinds of law about what you can and can't put in a contract. You can't just write whatever you want and then hold people to it. Why do you think they always have a clause that says that if part of the contract is found to be invalid, the rest still applies?
"1.5 Updates to Terms...
...We may make changes to the Terms from time to time, and if we do, we will notify you by revising the date at the top of the Terms and, in some cases, we may provide you with additional notice. Adobe will not make changes that have the effect of imposing additional fees or charges without providing additional notice. Any such changes will not apply to any dispute between you and Adobe arising prior to the date on which we posted the revised Terms incorporating such changes, or when the Terms otherwise become effective. You should look at the Terms regularly..."
"...Unless otherwise noted, the amended Terms will be effective immediately, and your continued use of our Services and Software confirm your acceptance of the changes. If you do not agree to the amended Terms, you must stop using our Services and Software and, if applicable, cancel your subscription..."