Multiple Prepayments for PO in 2013 - is it possible?

Hello all,
in NAV 2013 I've tried to post multiple prepayments for PO and it seems that it's not possible.
Is there any update on this or it's not achievable?
Example:
1) Create PO
2) On header select prepayment 20% and post prepayment -> Status is Pending Prepayment on document.
3) Post Payment journal against that prepayment invoice
4) On that PO still "Pending Prepayment".
5) Now I want to change % from 20 to 40. System asks to open document, opening document and trying to change % -> error message "must be 20% when the Prepayment Invoice has already been posted"

Checked the code and it seems that there are no such functionality:

IF ("Prepmt. Amt. Inv." <> 0) AND
   ("Prepayment %" <> xRec."Prepayment %")
THEN
  FIELDERROR("Prepayment %",STRSUBSTNO(Text045,xRec."Prepayment %"));

So there is only 1 prepayment can be done? Checked 2013 Original and 2013 Latest codebase. Seems R2 has same thing.

Any advice?

Thanks