Voucher Match error message: PO line is being changed from either a quantity line or lump sum line..

On a 2 way PO voucher match. we get an error when selecting the orders to match, details below.

Cause - The purchase order line is being changed from either a quantity line to a lump sum line or a lump sum line to a quantity line.

Resolution - Do not change the type of line. If the line is not defined as either a quantity or lump sum, then cancel the line and add a new line.

more info, this is an old transaction. more than 4 months old and cancelling the line is probably not a good idea.  Please help where to look and suggest solutions.  Thanks