I almost did the same thing - put snows on my 189s. I actually went as far as buying the tires from Gary and waiting till early December to put them on. After I found out that it would cost $50/wheel to swap out the RFTs with winter Blizzaks, I went ahead and return the tires to Tire Rack and bought a set of dedicated snows. I figure why spend $200+ twice a year to swap out tires when I can spend $1200 once and just pay $20 to swap out winter/summer set.
Not sure why it didn't occur to me before since I swap out the wheels on my X5 each season. Although always having 2 full sets of wheels in the garage takes up a lot of space.