Nope. The 420AT is fuel injected and has, IIRC, 14 electrical plugs on the motor. Wiring will be different, frame will be different, driveshafts won't be in the right place, etc etc.
I guess you COULD do it, but the cost would probably be more than MSRP on a new DCT Rancher.
Replace the clutches on your 400 and keep riding it. If you've gotten 4K miles out of the original clutches just put new ones in and go another 4K miles.
Now you MIGHT be able to swap a 350 Rancher footshift motor into that 400 without too much trouble. Still won't be bolt in, but would be much more likely to be workable. You'd still have to replace all the wiring and electronic crap with 350 stuff though.