I have a 20 inch Trek 7.2FX 20 inch frame bicycle which is my work horse and it's a tadd too small for me. I have the opportunity to purchase a Trek 7.5FX 22.5 inch frame only. Can I use the same fork from my existing 20 inch frame to put on the 22.5 inch frame?
If they're both current models, they both have a 1 1/8 inch threadless headset, so the fork should transfer as long as the fork's steerer tube is long enough to accommodate the bigger frame's longer headtube.
Can you take a few measurements before committing? Measure your current headtube, the bigger frame's headtube, and any headset spacers (above or below the stem). So long as the bigger frame's headtube doesn't exceed your current headtube plus spacers, you should be OK.
Looking at the current model year, the 22.5" 7.5 differs in at least 2 ways from the 20" 7.2 that may be relevant. (The same as between the 20" and 22.5" 7.2 FX)
The Head tube is 19 cm instead of 16 cm. If you have at least 3 cm of spacers to remove, this presents no problem. Based on my 20" 7.3, I think it would be close (I don't have the bike close by to check)
The other difference is that the trail listed changes from 7.0 to 6.2 I don't know how much difference this would make, but it is the only other relevant difference I noticed.