Design-Build usually results in a contract between the design-build team (engineer-architect-general contractor) and the owner. The contract is usually based on a specification that defines the physical and performance requirements of the finished product. It doesn't say much about HOW it is to be built.
Cost-bearing changes are usually based on changes to the specification. If the owner wants something beyond what is in the specification, he expects to negotiate a change order.
The sub-trades may have some kind of agreement/contract with the general contractor, but usually don't have a separate contract with the owner. Cost-bearing changes to the sub-trade contractors are based on their contract arrangement with the design-build team. You can't have a design-build team agreeing to have you do something that you don't get paid for if it is not in your contract. A couple of guidelines are:
1. If the design-build team gets paid for a change order, then the subs doing the work on that change ALWAYS get paid for their part of that work.
2. If there is an oversight by the design-build team that requires more work that is not covered by your contract, then you should get paid and they should eat the cost. That is why they have contingency costs in their bid.
