So, I bought the train sim world digital deluxe edition, will I get the the train sim world 2020 deluxe edition, or will I have to repurchase that?
Why? How many other games do you get the new deluxe edition just because you had the last? In fact, how many other games release a new one, but you can still keep your existing one and get an upgrade. With the option to purchase just the new routes that you want, and at a reduced cost? None on console as far as I am aware! If that was more common then we wouldn't get a new COD game every 5 minutes!
You will keep everything from the original deluxe edition, so all routes (NEC, RT, GWR) and access to the extra loco. You will be able to buy all routes included in TSW 2020 at a discounted price. The new US route will be available to you like it was a new route DLC. So depending on how many of the DLCs you own and actually want to have, it might make sense to go for the new deluxe version, or just pick up the routes you don't have.