If you want a 140mm bike, don't upgrade the Fuel EX to that level (not designed for it, etc.) but if you are set on it, 20mm is not that much of a difference and the worst that could happen is the headtube could shear off the frame, but for such a small difference that is unlikely.
If you are going 140mm, look into the RockShox Pike 29 140.
i would advise against changing to a fork that gives a geo that the bike was not designed around - you just don't know how it'll ride/handle afterwards. I suspect that if you did have issues and Trek knew of the change, you may have issues too.
I put a 2013 RS Revelation RCT3 120mm on my Fuel Ex 5 and have been very happy with the results.
A forum community dedicated to Mountain Bike owners and enthusiasts. Come join the discussion about bike parts, components, deals, performance, modifications, classifieds, trails, troubleshooting, maintenance, and more!