You can add another to replace the one you had. So if you had 3 FP's, used 2, and then the 3rd ride goes down, you get an anytime FP and the 3rd FP disappears. You can then add a new 3rd FP. That 3rd FP would still follow the tiered restrictions that the original had. Now if the ride happens to come back up and there happens to be availability you could add it back as your 3rd FP and still have the anytime FP.