I have a contract with a trucking company in Montana, can the new owner make changes to that contract without my consent?

Assuming that this is the same corporation and just a new owner, or that your contract was assigned to a new owner, your contract remains valid.  You have no obligation to accept any changes.  HOWEVER, does the contract contain any restrictions on the trucking company's right to fire you?  If not, the trucking company is within its rights to fire you unless you accept the new payment structure.  For example, if your contract provides that you will haul three loads a week for $500 per load until the end of 2015, with no means of terminating the agreement, then you have the right to haul 3 loads a week at $500 per load until the end of 2015.  But if your agreement only says that you will be paid $500 per load for all loads you haul, the trucking company has to pay you $500 for any load you haul, but does not (absent factors not mentioned in your question) have any obligation to use you to haul anything, so that if you don't agree to different pricing, the company doesn't have to use you.