IANAL, but in the law there's a thing which is called Contra proferentem: https://en.wikipedia.org/wiki/Contra_proferentem Basically, it means that ambiguous terms are interpreted against the contract drafter.
Edit: spelling
Of course, a counterclaim for existing owners would be that the product they purchased was licensed for DC usage at time the contract was entered into, and so the original rights cannot be unilaterally revoked without consent.