As a fan since the beginning I'm not really concerned as of yet, I hope that is is an opportunity to rebalance some pokemon and revamp the pokedex itself.
Revisting things like the Nidoran genders and revamping them into one pokemon, introducing the mega forms as a new evolution (for some) and new forms (for others), changing Espeon, Umbreon, Leafeon, Glaceon and Sylveon to evolve by their respective stones. Change the Dex order so that families are grouped together rather than being separated by different generations. Stuff like that
As well as balancing types like Ice and reworking pokemon star distribution and abilities to make them work better.
The leaks suggest
So they are at least going back and changing up some pokemon