Technically the answer is yes, however,  just because you dont have enough funds of the right appropriation is not a legal reason to use the wrong color of money.  Your comptroller may find this proposed action a violation of the Purpose Statute (Misappropriation Act).  Since you used O&M money last year, not having enough and trying to use another appropriation (RDT&E) this year would be a red flag to the comptrollers office.  Replacement parts are not RDT&E, and you could be pressing up against the Bona Fide Needs rule. That  rule requires funds to be used only for needs in the year(s) of the appropriations obligation period. So you would need to explain how a one year O&M requirement somehow became a 2 year RDT&E requirement.  This proposed action could also trigger a 1517 review.  Discuss this with your comptroller to make a determination on what is the proper next step. They will  have to get involved no matter what action you take.