APPROVED PO GL-PERIOD CHANGING

 hi Experts, the PO is approved by all authorities now while making an invioce we have the PO which is approved having the GL DATE of period for which no budget is availiable therefore now i am looking to change the GL period date .. offcourse it is an ecumbrance perid ..!!
what i am thiking for it is to unrerve the approved PO do make changes of GL ecumbaraed period and then click on the approve button ..!!
tell me is it possible or not ???