Most contracts that don't have a "bond" have an early termination clause, generally 1-3 months. Either party may terminate "early". Pay is generally not renegotiable until you renew your contract, unless you have some big brass ones and are willing to quit and the company really thinks you are the greatest pilot on earth. This hasn't happened to me yet. Ha ha.
The only time pay is generally negotiable is when you work for Rishworth. At some point, they will decide that you are receiving too much money, and will unilaterally decide to not give you as much as your contract states. I wouldn't call this negotiating, I would call this criminal. This HAS happened to me.
There are so many different contracts, the terms are all over the map. The one thing to remember is that you are your own "business". Negotiate carefully, and be careful what you wish for.