The first game that comes to mind for me is Civilizations 4. I’ve probably spent hundreds of hours playing but after getting used to 5 and 6 I have a really hard time going back. Going back and forth between 5 and 6 I need to rethink some strategies but with 4 I feel like I need to rethink everything. I don’t know if it’s because of the tech tree layout in combination with the civics system, things like unit stacking, or maybe just a bunch of little things but it takes me a while to readjust.
Any of the fallout/elder scrolls series. I remember whiling so many hours away in them as a kid and having a great time, and now any time I pick them up it just feels like a second job, looting and managing inventory ad nauseum.