I strongly suggest you speak with the Business Financial Manager, and/or Legal Office for additional guidance.  The first place to look is at the Budget Exhibits for this particular effort. Did you include this in-scope modification in the request for funding in the budget exhibits?  If you did then using current year funds would be fine, if you did not include this modification funding in the budget exhibits then what effort/program/task are you taking funds from to fund this in-scope modification?  There is no definitive DOD guidance that pertains to not using prior years funding for a modification if you have used current year funding.  However, the US Air Force may have more specific guidance that requires you to use prior year funds rather than current year funds for an in-scope contract modification.  The more common sense approach would be to use prior year funds (if available) for an in-scope contract modification in order to persever the current year funds for the programs/tasks/efforts that were shown in the Budget Exhibits.