Regarding the "correct contractually" aspect of your question— Since realigning the funds from one CLIN to another changes the parties' obligations under the contract, a bilateral modification would be needed to obtain the contractor's consent.

Funds are obligated against CLINs, so they have to be deobligated and then reobligated. That said, the DoD Financial Management Regulation prohibits the reobligation of expired funds (with a possible exception for contract terminations).