That is my question 
Post from Kris:
Here are the vertical clearances for KH frames, measured from axle to underside of crown:
KH20: 268.5 mm
KH24: 338.5 mm
KH27.5: 383.5 mm
KH29: 401.5 mm
KH36: 463.5 mm
Horizontal clearance is mentioned on the KH home page for each frame. Just need to check your wheel dimensions now…
Kris also mentions the frame clearances in this post: "Official" 29er Tire Reviews Thread
The clearances are measured at the widest part of a 27.5 tire so there may be a couple of more mm’s with a 26" wheel (due to very slightly shorter rim) - so if you have the 2017+ frame maybe you’d get 98mm of clearance.
Note that Kris added the 2mm to the width of the frame from 2106-2017 to better support the Duro Crux 3.25" tire that he measures at 81mm on his 55mm rim. From that I take it that 15mm of clearance is better than 13mm to allow for tire variation and rim trueness.
I have a bike with a 27.5x3.8 tire (Terrene Cake Eater) on a 52mm rim that measures 93mm wide at the knobs. Given that, I’m thinking even a something like a 26x3.8 on a 50mm rim would be extremely tight and likely to rub.
If you want to cross from plus size tires to fat, a frame designed for that is probably the way to go.