Does anyone know whether we will be able to use our existing DLC with the Gamepass version or are we restricted to the three base routes?
Judging by how Game Pass licenses work, I'd say we'll be able to use our already purchased TSW2 DLC with TSW 3 on Game Pass. The system basically doesn't care how you can access the base game. If you have the right to play it, you can use all of its features. So if you have a Game Pass membership, in the digital world it's the same as if you'd purchased it. You have access, that's what's important. So yes, the DLC will be downloadable under TSW 3 as well.
You will be able to use your Purchased TSW2 Routes but the Core TSW2 Routes that you have with Game Pass you won't be able to transfer over to TSW3, you will need to Purchase TSW2 Core for it to transfer over to TSW3 if you would still like to use the Core TSW2 Routes.
So does that mean if I already own tsw2 I can still use all of my dlc on the gamepass and I get the extended version of sehs
TSW2 Core on Game Pass - The Core Routes Bakerloo, Sand Patch and Koln , won't transfer you will need to buy TSW2 Core to get these routes in the TSW3 Game Pass. TSW2 Purchased - If you own any other other routes and locos that isn't the core 3 TSW2 Routes, these will transfer. Hope it Makes it Easier so yes if you own SEHS you will get the upgraded on the game pass TSW3 version