Once you have signed a contract, all of the vital decisions are theirs. They hold all the trump cards. The myth of course, is that if they violate the terms of the contract you can sue them. It is a myth because it takes a lot of money to sue an individual successfully and a ton of money to sue a company successfully and that brings me to the second hard truth about companies. No company is ever going to pay you enough money to sue them successfully.
“Dave Sim, [1]” Address to Pro Con '93.
It's important to read that. Never mind if you aren't in the Comics Industry; what he says applies to any intellectual property (IP) you may create.