Don't make the change until you get a good offer -- and they will pop up repeatedly.

In September, I was sent this offer. It gave me 5000 StarPoints for signing up and an additional 1000 for each of my first six stays at a Starwood property within a year.

The link is still up and there is nothing on it, or in my e-mail, that says it is targetted so you might want to try it.

(One StarPoint = one SkyMile and if you redeem 20,000 StarPoints you get 25,000 SkyMiles.)