i have done this on other bikes with great suscess. (21 to a 16)
i have just purchase a 16 inch rim, and had it powder coated black. tuesday my new SS spokes came in, so now it time to remove the 19 inch, and brake everything down. i am having the orginal spool laced to the 16 inch rim. this way there is no problems with bearings, spacers or the new SB size axle. i have seen one other done this way, it really adds a lot of "beef" to the looks of the street bob.