DLR will let you in on the first day you use your AP, even if it's a blockout day. So if you're only going for one day, you don't have to spend the time upgrading, but if you are going for more than one day, and some of the days would be blocked out, then buy the multi-day ticket and upgrade. They say you cannot upgrade a more expensive multi-day ticket for a lower priced AP. I'm guessing it's specifically to keep people from trying to get around the blackout days during the busier seasons.
Hope that helped.