>>33535378>>33535352>new Pokemon in the middle of a generation
It's just like having new forms of Pokemon in the middle of a generation. Pretty sure Kyurem-B and Kyurem-W were incompatible with Black and White.
My guess is that these two new Pokemon will be given new space in the PokeDex, likely after the existing Ultra Beasts, and then Marshadow and the other event legendaries would be pushed back a bit. This would effectively retcon Sun and Moon out of existence, and any following games would use this new ordering of the PokeDex.