For physical copies, you will be able to download the upgraded version when it becomes available. The disc must be inserted to launch both the current-gen and next-gen versions of the game.
Next-gen upgrades are not available for physical versions of the game if you have upgraded to a discless console. It is also not possible to upgrade from a digital current-gen edition to a physical next-gen edition.
Once you have completed the upgrade, you will still be able to play your game on your PlayStation 4 or Xbox One.
Any additional digital content unlocked on the current-gen version of the game will be transferred to the next-gen version, including Season Passes, pre-order offers, and Ubisoft Club rewards.