Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
Programs: BA Lifetime Gold; Flying Blue Life Platinum; LH Sen.; Hilton Diamond; Kemal Kebabs Prized Customer
Posts: 63,855
You can't do it online.
You can in theory do it by calling in and removing the POUG (and losing the fee along the way). It's possible the POUG can carry through to the new flight but you certainly count on it, there may be a fare difference on the POUG component, and you can't tell in advance. This changed a year or two back.
I have to say, it really is best to avoid adding a POUG unless you are 100% sure of taking the planned flight.