I modified the t6poia1 form to show additional field (line item $$ extension) and saved it as t6poia3.  Printing the open PO's PO-I-A the form choice appears but choosing  one other  than t6poia1 still gives me t6poia1.  When I saved my #3 to #1, I could see my changes.  
 
Could you make the choice here for real?
 
PS Ditto with PO-I-B.