Unless the rules have changed, you should not have been allowed to replace BWV points in that March reservation with VGF points if the reservation was made with BWV points during the 11-month window.The MS person you dealt with either did not know the rules, or the reservations was still separately avaialble when the change was made, or the MS person decided to make a special exception for you.
The only other recognized exception I am aware to that inability to use other resort points for a reservation made 11-months out is that you can add dates to a reservation using other resort points at 7-months out. A reason for the general rule is that, if you were allowed to just switch points to those from another resort at 7-months out, there would be many members owning, for example, just enough expensive VGF points for making a reservation at 11-months out, and then switching all the points to low-cost SSR points at 7-months out, and then banking the VGF points into the next use year to use for another trip made at 11-months out.