What would be involved if you installed 226 9mm upper on 226 dak .40 frame, would weapon function? Any other issues?

P226 frames are the same if they are of the same relative age. If you have a stainless steel 9mm slide and it fits on the DAK frame, it should work fine.

I have swapped a stainless 9mm P226 slide on my P226 .357 DAK frame and it functioned just fine for 500+ rounds.