I will share this with you and maybe it will make you feel better. We've been to WDW around 11 times and about everytime once we get there we want to stay longer than we booked. We always check at the front desk of the resort we are at and we have always been able to add additional nights with no problem, even during the busy times. Now we have had to change rooms at the same resort, because of large groups that may have booked the current room we were in and they needed it on the extra nights we added. But we've still been able to add those nights.
So, if you can't get them before check in, just check once you get there and then keep checking each day with the front desk.