technology update...

I have raised a PO, say for 100 Rs. for an item 'X', but then due to some technological reason the product gets updated and I want to increase the amount of PO say 150 Rs., any revision will mean that the approval will be again required for that revision, but I don't want to put the PO for approval as it has already been approved. (Its a case study given to me)...please help...