Newmen has manufactured some keyboards with a mixture of old-generation Kailh Red switches (the ones with a weight intermediate between red and black) and actual MX Blacks. Cherry switches on WASD and the arrows, Kailhs everywhere else. These got sold under the Genius and Shogun Bros brands, and maybe one or two others. Of course there was a difference in feel, but then, in that case, there wasn't any real effort being made to make the two identical.
As for replacing a newer-generation Kailh with a regular MX, it's "compatible", so you can do it, but it might not feel entirely identical, especially if the Kailhs are of the kind designed for a higher activation point. On the other hand, there are some pretty cheap keyboards using Kailh switches, so when we get to the point where Kailh boards are starting to get sold used or for parts, it might not be too painful on the wallet to get a donor board to transfer from. Also, at least some brands are offering long warranties on their Kailh boards to increase confidence, so it won't be your problem to worry about for a while (unless you void the warranty).