You can do this at any time. If you decide (and the rest of your league agrees) that you want to make your league a keeper league starting now, the commissioner of your league can set the option next year using the commissioner controls to set the number of keepers for your league. He can set it up so you can either submit your keepers to the commissioner and he can manually put everyones keepers in, or he can set it up so each team can submit their own keepers. If you do it this year, I suggest you set the keeper limit to a low number (3 or 4) that way anyone that was playing under the assumption that it wasn't a keeper league will still be able to have a good team in the future. You can always raise the number of keepers per team the following year.