Regardless if it is still law I imagine you could get out of the contract in the event they pulled this. Contract Law is pretty clear on what is allowed within contract negotiation and having one party modify the contract agreement (regardless of the B.S. clause of modifications can occur without consent) is typically not widely accepted.
Having said that your contract with the ISP is worth roughly $500 or more depending on when you entered the contract. If you started to lean on them with a "legal" threat of contract termination and taking it to the courts blah blah blah they'll most likely voluntarily terminate the contract because its not worth their time/effort.