Modifying a 2 way complex PO into a 3 way complex PO (12.1.3)

Till last month, we were having many complex PO which were 2 way matched and invoice was being created against them and settled.

Now, we have gone live with work confirmation functionality and want to use this for all complex POs.

1) Is there a way to convert already created 2 way complex PO into a 3 way complex PO. These POs already have Amount Billed for partial amount and remaining amount now needs to be handled as 3 way.

2) What will be accounting behaviour if we are able to change it ?

Thanks, Vishal.