Well, I just "transitioned" a horse from Western to English. I can say the process took longer than I thought it would. Like CessBee said, make sure you give your horse plenty of time to get used to the feel of the saddle, the lightness of it, and the way it rides on him. Also, when riding him for the first few times in the English saddle, it is best to just walk him so he can understand how the rider is supposed to move with him.
I was stupid enough to just throw my saddle on the horse and hop right on. Which caused my horse to throw me many a times.
Red Money Maker (Red) - 2004 Sorrel QH Gelding