A:Answer According to Ubisoft's PS4 -> PS5 upgrade faq, Yes. You'll be able to play on either console but you'll need to have the disc in to play.
"ONCE AN ELIGIBLE PLAYSTATION 4 GAME HAS BEEN UPGRADED TO PLAYSTATION 5, CAN I PLAY IT ON PLAYSTATION 4 STILL?
Yes. If your game is eligible to be upgraded to a next generation console and you go through with the upgrade from PlayStation 4 to PlayStation 5, you will still be able to play the game on your PlayStation 4 using your original copy of the game. Note that for physical versions of the game, you’ll need to have the disc inserted in the tray to proceed."