How Congress could reshape a proposed BBGN

Could a new US administration modify the proposed battleship hulls?

Yes, but the ease of changing them would depend on the wording of the appropriations law, and Congress could authorize a different design through another bill.

Technical possibility remains subject to legislative control. An administration may have room to alter a ship within an existing appropriation, but tightly written requirements can constrain those changes. Congress can also pass a new appropriation, making the final design a product of both naval engineering and budget law.