I've used Holds in the past and I think I have some understanding how BA's fare buckets move. If you don't complete the hold the availability will return to inventory when they lapse, or in the seconds that follow. However BA's dynamic inventory processes means that you cannot assume that the bucket will restore by the same amount of seats at that point. There is also a degree day system which works on anticipated demand versus actual versus historical. For example you could accept the hold and then a week later additional inventory in that bucket will appear simply because anticipated sales were not met, and so they need to encourage sales to fill the aircraft. It really is a gamble and trying to outwit BA's revenue management processes is a mug's game. This is all a general statement, without flight details I am just guessing here.

You are better off giving BA a call and seeing if an agent can get you the desired outcome, however as I understand it AARP can only be done online, and an agent can't fix that.