If a proposal is or has been selected for the design, development, construction, maintenance, or operation of transportation systems or facilities under this chapter, subsequent agreements may be made to implement portions of the proposal that modify the proposal or that do not incorporate all the features of the proposal. Any such modified agreement does not require the solicitation or consideration of additional proposals for all or any portion of the services rendered under that modified agreement. Modified agreements may provide for the reimbursement of expenses and fees incurred under earlier agreements.