Quote Originally Posted by Eurogamer
[...] One potential workaround I've seen suggested in various places is to change your console's region to Turkey, which has then made the upgrade option appear for some people. [UPDATE: Specifically, I'm told, to create a second PSN account set to Turkey to download the digital edition which then runs in English.] [...]

Today, Ubisoft sent through the following instructions to try and get the upgrade (TL;DR, you should select "view product" and check in the game's hub) - though I've also heard from affected users saying "view product" isn't accessible to them.

Here's Ubisoft's suggestion:
- Insert the eligible PS4 game disc.
- Go to the game hub for the PS4 game. Users must select "VIEW PRODUCT"
- Select the upgrade offer on the game hub and review the upgrade offer.
- Select Download (or purchase with the discount price) to download/purchase content.
- After the download is complete, you can play the PS5 digital game by inserting the PS4 game disc. You will need to keep the disc inserted each time to play the game. [...]
Ubisoft investigating missing Watch Dogs Legion, Assassin's Creed Valhalla PS4 upgrades on PS5 • Eurogamer.net