I read through all 15 pages of this thread to try to help anyone that asked the question about whether or not they will be issued a replacement FP if they can not ride FoP because I asked this exact question on the official Pandora thread 3 times and it got buried without an answer (probably because no one had it happen to them yet).
We just got back from our trip to Pandora a few days ago. I had gotten our FP for FoP when they first came out before there were any rumors about the size/shape restrictions for this ride.
I tried the test seat. It was awkward, kids playing on it, a large group of people all gathered around staring at it. I just patiently waited my turn and then kind of had to push myself in because kids kept jumping on and off it. I did not fit. If you want my exact measurements you can PM me but I prefer to call myself Ursula-shaped to help you get an idea: bigger, busty, larger bottom. I used to compete in weightlifting at the state level so for me, my calves were most of the issue. The back restraint wasn't even close before it wouldn't budge anymore because my calves are so large. I tried several different positions to no avail. When I asked to have my FP replaced so I didn't have to wait in the line to NOT ride the ride (the FP line was about 30 minutes at the time) they told me I
had to go through the line to get the replacement.
I waited in the line and when I got to the vehicle room the CM was very helpful. I told her I knew I wasn't going to fit but was told that I had to go through the line to get my replacement FP. She said "I'm willing to try if you are!" and I decided to give it another try. I still did not fit but I learned one additional piece of information I thought I would share here...
The seat on the actual ride vehicle is some sort of rubberized material that makes it virtually impossible to 'slide' forward. From what I can recall, the test seat did not have this rubbery material because I was able to slide all the way forward on the test seat. This may be one reason why they are telling people that "Fitting on the test seat is no guarantee that you'll fit on the actual ride vehicle."
As an aside, I happened to be seat #3 of 16 seats in our section so for people saying that they aren't seeing anyone being told that they "can't ride" you may not be aware of how many people can't ride because each room only has 8 seats and I only walked past 2 people to exit the ride area.
Once I exited the ride area, I chatted with another very nice CM who I told that I was not able to fit so I couldn't ride and I was disappointed because I had used one of my Fastpasses for it. I assumed that she would issue me a replacement FP at that point but she continued to chat about the interesting details that you may not notice in Pandora. About that time the ride cleared out and I stood there waiting thinking maybe I needed to ask the CM that saw me not fit on the ride for the replacement but she never came into the exit area. So I finally just asked "Who should I talk to about getting a replacement Fastpass?" And she said "Let me scan your magic band." She did that for me and my sister and that was the replacement.
So long story short: If they don't volunteer it, just ask for the replacement.
And for those that don't end up fitting on FoP, here is a consolation: Obviously this won't be true for everyone, but much to my surprise, my sister said she far preferred Na'vi River Journey to Flight of Passage. Enjoy your visit anyway, I sure did!!!