Did you originally book your FP when they had changed Mission Space to a Tier II and so you have what now looks like two Tier I FPs? I have that situation right now. I booked Test Track as my Tier I and then chose Mission Space as one of my Tier II. Now, they have moved MS back to Tier I and so it appears as though I have two Tier I FPs. It won't let me change the time of either TT or MS until I change the other one to an "actual" Tier II (even though in my head, MS is a Tier II because that's what it was when I booked). I wonder if that's why you can't modify your Tier I. Does that make sense?