I would say banning slavery says we have right to employment as a institution. We dont have specific rights to specific employment.
If you want to argue a corporation is a person, then the employs should have a similar standing. Breach of employment contract would be similar to breaking any other contract. And any changes to that contract would either require specifc wording in the contract, or renegotiation. If it cant be solved between the two, involve the courts. Until that time the original contract holds. Again, unless there is very specific wording in the contract one side doesnt get to just back out of it or change it.
If you dont like the terms of employment dont hire them. It's a two way street. It's not just on the employee to gtfo.