To say it another way -
If you are walking at 11 months, you only need to modify
a day before the 11 month window opens for the night you want to add. You can do it more often, but it really isn't necessary.
The only thing that could get in your way is if
DVC takes some of the villas you want out of inventory for planned maintenance. It's a very rare occurrence for that to happen before the 11 month booking window even opens.