I don’t have an SP404MK2 but I don’t think one would necessarily replace the other. There was some discussion in the SP404MK2 thread: Roland SP-404 Mk2
A key difference I see is that the FX suite on the SP404MK2 is probably better at affecting incoming audio (and has more types of FX though quality of them is your personal opinion). The punch-in FX steal the show for me on the KO2 but those won’t affect incoming audio. You can affect incoming audio with the other send FX (reverb, delay etc). KO2 looks immediate above all else.