I am pretty sure the rooms are assigned before you get there. In Jan, we had a standard room reserved (at the great AP rate of $89!). When we got there, we were assigned a city view in the Marina tower on a low floor so I smiled and practically begged for an upgrade cause it was my bday-lol. She did switch us to a room in Bonita on the 2nd floor overlooking the waterfall. I loved the room but later found out that it too was considered a "standard" room. There are several standard view rooms that look into the pool area but they are on low floors/blocked views. Even Sierra Tower has some "standard" views that look out at the pool or DTD but low floor/blocked view.
The next time, we went in March and paid for the view. I did not want to actually have to beg ever again-lol. From reading all of these boards, it seemed as though "free" upgrades were hard to come by, especially with all the 50th happenings and there were almost never any vacancies at the DLH. We had to book months in advance to even get into DLH! So that trip, we arrived a day early and ended up in a beautiful room on the 14th floor of PPH for the one night. I loved the room, the view was just spectacular so in the morning I called the desk and asked if we could arrange to stay and transfer our reservation over. It was about 8 am and the CM at PPH said it could possibly be worked out but we were already assigned a SUITE!! at DLH! OMG! (The CM was so cute and excited for us too and said she was sure we would want to keep our reservation at the DLH.) We were so excited and it was wonderful to actually get an upgrade free! We had booked with Expedia that time too because we got the $150 gift card deal.
We always tried to check in as early as possible believing that you would get a better room that way but after March, I am sure they are already assigned at least by 8am! . If you are checking in and it is early, they probably have more flexibility moving you because others haven't checked in yet. But if they are full, they are never going to move you to a "view" if someone else has already paid for the view.
FWIW, it drives me crazy to pay for and "request" a certain view and not get it. It seems as though everyone wants the Sierra Tower DTD view but I've read that room type "view" only makes up 20% of all the view room so not everyone that wants it will get it. I think DLH should have 3 tiers of "views" and charge accordingly-no view(city view) and pool view and resort (DTD) view. I just hate paying for the view and playing the room lottery to get the DTD view but love seeing the fireworks from the room. My kids nod off early so we spend a lot of time in the room and I love to people watch so I really want that view-lol. In June, we actually finally got the view and guess what, it was so darn noisy from ESPN Zone going all night long!! The music stopped at around 10 or 11 but the sports announcer noise went all night and woke me up at all different hours, one night 3 another night 5 am-ugh. Our door did not close tightly so that probably made a big difference. We go back again in Nov and again paid for the view but I still am not sure what view I want-lol.