dear:
solution 1: don't do any MM change. just do correction on Financial report. it is the easy way.
solution 2: create a new material number with half price; then do mvt 501 : no PO GR in current month. This action is a kind of inventory profit.
solution 3: do reversal for mvt 201 of the old spare part in current period. then do GI for half qty of the old spare part in current period.
please never go back to previous period.
good luck.