Starting with update 8.5, all upgrades from MK2 lvl 12 to MK3 will require a special token (or several tokens depending on the item type) instead of Au units.
The number of tokens you have will be displayed among other currencies at the top of the Upgrade screen.
Currently, you can receive Upgrade Tokens as a reward in Leaderboards and via Resource Operation Pass. Other sources where you can get the token from will be announced upon their introduction in the game.