XCF 350 suspension to FX 350 ???

Yes. The 21 has shorter suspension, so you have to adjust accordingly in the triple clamps, but it will be a direct swap...

It will fit, but since the 2021 has shorter suspension jts gonna ride like a chopper.
Monk its right, you can move the forks up in the clamps but maybe not enough to balance rhe bike

I actually just did this, moving suspension for my 18 TC 250 to a 21 300 TPI.

I put the 18 forks at the second line, like I had them on the old bike, set the shock sag and rode it. I expected it to be pretty tall in the front, but actually the bike feels very balanced to me.

I was running a 1mm longer link arm on the 18, so maybe I just like my bikes a little lower in the back?

21’s have .5mm longer link, different bell crank, and 6 mm less stroke on the shock. I believe the shock length is the same, but the stroke is slightly shorter. 21’s also have 10mm shorter forks (length not stroke).

So 🤷🏻‍♂️ I’d say put it on and just play around. Going to the new bike did make my forks feel too low in the stroke. Going to go to a 5.0 from a 4.9 and added 10mm of oil in the mean time. I think it’s just the front nature bias of the new style frame. Shock was pretty close and same spring worked.

Good luck!