Utilization of FAR Part for Modification

I am working on drafting a modification to an existing line-haul contract. The base contract calls for trailer types to attach to hauling trucks. The modification is to change up the trailer type numbers (i.e. fewer flatbeds and more lowboys). We are not requesting additional funding as we are using the price differences to support addition to CLINS (removing CLIN requirement balance and reallocating balance to a different CLIN). 

What is the appropriate documentation to complete this requirement? Is a J&A appropriate because we are the source to the existing contractor? Or is it an in-scope determination?

Please let me know if this needs further explanation.