I actually dont know the specifics of it. Apparently, United and a few other carriers sent something out to their customers stating that if they enroll in the program, and automatically roll their miles over into the airline program, Starwood comps them to gold. Gold gives you a 4pm late check out, a 50% bonus on points (3 points per dollar instead of 2) and an upgrade to a "specialty room" basically a better view or location. Personally, keeping the points in SPG will be much better for you. I dont know if the 20,000 mile bonus applies.

Besides the mailing, I dont know how the program gets started. Try calling SPG to see if they can do it, or call your FF program.