The funny thing is by forcing you to agree to the new terms, the contract can be challenged since one side was coerced to sign it (and didn't get a chance to sign in voluntarily!).

US courts tend to favor corporations over end users, so there's still a strong chance a judge will throw the case out anyway, but because this is such an act of bad faith in US contract law, a judge might also rule in the end-user's favor just to make an example out of Blizzard for being such a dick.

PS: Steam did this a long time ago. I've never had any disagreements with Steam but some folks have. I don't know if anyone's had the account bricked, which Blizzard, EA and Ubisoft have done.