Telefonica (Spanish PTT) tried to do same thing here. They sold a lot of adsl and after tried to limit b/w and charge for each extra MB.
They couldnt sustain that in a court (changing on going contracts) so:

1) old contracts stay as they are (mine is one of those)
2) new contracts have b/w restrictions. And more than yours (top limit is 6GB/Month here)

They cant assign new clauses to a service without a contract re-signed. Send a letter to them telling that you dont agree and you wont pay anything extra until the end of contract. If they refuse , go to a court.

I think the courts there are better than ours.