Worry not, you signed a contract. If they change the terms of the contract, demand they keep your service unchanged or cancel as a breach of contract.
As if I would be allowed to use a lawsuit anyway, I'm sure I'd have to use mandatory binding arbitration.
Should I switch to the carrier I left because they have no coverage? The one that's going bankrupt? The other one that's probably going bankrupt tomorrow that tried to merge with them? Or maybe one of the MVNOs that's really just one of the above?
There isn't much choice available.