According to an article I read, and posted here.

CO will be trialing these winglets of 8 of their 737NG’s initially. If they prove their worth, the entire 737 fleet could be retro fitted.

Also, 8 of CO’s 757 will go through the same trials, with the rest of the fleet following suite should the winglets prove a benefit to the 757’s performance.

Dartie: CO are very happy with their 757’s. They will be around for plenty of years to come yet. I myself get my first flight with a CO and with one of ther many 757s in september… looking forward to it 🙂