No, they aren't replacing Ash or setting it up. You can have a few points for that.
1. Pikachu is the Pokemon Mascot
2. New Protagonist will have a Pikachu as its main Pokemon because of it as well
3. As such, there is no point in replacing Ash and Pikachu
4. The new Protag will likely get the starters and the popular mons as well, and considering Ash already has those, there is no point for the new Protag to also get those mons, but they have to be marketed somehow.
5. You cannot have Team Rocket without Ash, just like you cannot have Ash without Team Rocket, so its essentially changing 5 characters (6 if you count Wobbuffett) instead of 2 and this is what many people don't realize.
6. All the character and Pokemon build-up was pointless in all the 20 years of the anime.
7. At this point, its too late to replace Ash. If you wanted to replace him, they should have done so after Sinnoh, then have a new Protag for Hoenn and Sinnoh, replace that one with a Protag for Unova and Kalos and then one for Alola and Galar and start gen 9 with a fresh new Protag again.