I am wondering if anyone has written a macro or knows of a 3rd party tool or something that would allow you to change the tax group on a PO after you have added a line item.  The fact that this can be changed at the AP invoice level would suggest to me that it should be possible but PO's doesn't allow it.  In our situation the individuals doing the receipt / invoice all in one step can't ever balance to the suppliers invoice in situations where the tax group has been assigned wrong (ie there is a purchase from a vendor that isn't appropriate for the default tax group that the vendor has been assigned.  

1. create a PO

2.  Vendor's default tax group is assigned

3.  Add a line item

*** from that point you can not change the tax group without removing all the lines of detail up until it sent to AP - then you can change the tax group.