A little bit of bad news - good news:
The bad - To my surprise Gamemaker forced me into the latest release ( = I can't return my project to previous versions). The plan was to schedule march/june to make that transition and deal with the fallout, but I guess that's been pulled forwards.
The good - Gamemaker improved the automated transition and I can at least launch and play without instant errors. (The upgrade attempt last year ended in a nightmare). There are a number of visuals issues now (likely due to a specific outdated feature I was meaning to get rid of anyway) and I'll still have to comb a massive number of scripts, but it looks WAY better than expected.

So all in all I'll have to postpone finishing the latest dungeon for now. At least untill the most glaring visual hickups are dealt with. I'll also release the current state after the weekend for those that want to help with bughunting new issues after this update.
EDIT: Uuuhm. Found out why I didn't remove that one outdated feature earlier. It's used literally thousands of times. Welp, it's going to be long weekend.
