OK thanks for clearing that up.There is no Steam UWP export (and 1.4 has never had UWP on Steam for purchase), so there is no upgrade discount for that purchase path. You can still get the discount on the YYG website and then apply it to the Steam version afterwards.
It's a shame though because purchasing through YYG works out £55 more expensive than if you could do it through Steam (based on the Mobile export Steam price which is the same price as UWP). It's not a massive amount I admit, but it's not an insignificant amount either.
Still, thanks for saving me some time messing with the account linking and wondering why it wasn't updating!