As far as I know, Congress appropriates money in the defense budget. I don’t think Congress then has a say in how those funds are spent - i.e., do we build an aircraft carrier, a new military base, or a border wall, for example. So all they would have to do is move some money around.
Only issue MAY be is the spending authority for the funds. Just because funds have been “appropriated” doesn’t mean there is the requisite spending authority...I think. In my state government days I learned about that think I had “x” dollars in my budget to spend on a project, but I was advised even though the funds had been “appropriated” by the legislature, the authority to spend them had not. I don’t know if it is the same way at fed gov.