Hi,
Before updating tax code for all open PO's- you need to check tax cod nature(( non-deductable or deductable) and status of all open PO's( 1. open PO's at PO level, 2.open PO's at GR level, 3. open PO's at invoice level).
Once you confirm all with tax nature, you can use BAPI or LSMW tool to update tax code for all your open PO's.
Regards,
Biju K