Yes keep calling-you never know.
10 years ago (wow-has it really been that long!

) I had alot of trouble getting rooms for our entire trip. I must have made 20-30 calls. At one point I had a room for the first few nights and the last nights with a 1 day gap in between. Luckily I was able to book that missing night before we left-otherwise we would have been sleeping in the van.
And last year, I had added another room to our ressie (in time for
free dining) but wasn't able to get the first night, and the last night, of our stay. It wasn't a big deal the first night as it was just 3A & 1K in one room, but 1/2 way through our trip my DNiece25 flew in to surprise my DD8 and that meant we would be 4A and 1K in one room on the last night along with all of our luggage!

I wasn't able to get that last night before we left but on the night before we would have had to check out of that one room the front desk called and said we could have the extra night, and keep the same (connected) room. Yipeeee!
So, my point it keep calling, plans do change. If you are flying down, it's a little trickier of course. But you can always stay

off property those first 2 nights.
Just a few days ago, after making my airline ressies, I called and changed our ressie at POP. I didn't add any days, but I did arrive one day earlier, and leave one day earlier. However, last year I did add days to my existing (Free
DDP) ressie with no problem.
One quick thought, before even calling go online and check to see if any rooms are available. That's what I did before I called POP this week. I let the CM on the phone know that I had already checked and rooms were available on Sept. 1st.
Hope this helps!
I would keep calling/checking until the last minute!