This is a really important point so you don't loose the benefit.
You can upgrade it anyday that your PH is still good. That would be for your case upto when you leave on the 4th day.
To upgrade any ticket the amount you paid for that ticket is credited to the purchase of your upgrade wether to another ticket or AP. The first day you used your original ticket will be the issue date of you new pass.
Also I would do this in Greetings From Califonia in DCA, it runs from the golden gate bridge to the end of the street. The AP desk is about halfway down on the streetside of the store.