That is usually a time of year when BWV is easier to get at 7 months than many other times of the year; however, you should, as you plan, reserve at home resort at 11 months to assure a room and then attempt to make the switch at 7 months out.
Also, it appears you want to switch for only part of the days of your trip. In that case, you need to know the following trick. When you make your ressie at your home resort, make two separate reservations and don't have them linked yet -- one for the time you actually intend to stay at your home resort and one for the time you hope to switch to BWV. The reason: if you make one ressie for the entire time at your home resort and then just try to switch some of the days, MS will require you to first cancel the original ressie entirely and then you need to get a new ressie both at BWV and for the time at your home resort, and whether you can get both those ressies will be subject to availability (and any existing wait list) at the 7 month window. In other words, you could lose your home resort time doing it that way. Thus, make two separate reservations at your home resort for the different days and then you need only cancel the one for the particular days you want to switch to BWV.