Went back to make a couple changes to the mod I did several years ago and tried to add an overlay. Made the wallgroup polygon just fine, checked it with floodfill, looked fine, loaded the alternate tileset, looked OK, clicked on Make Overlay and the popup came up saying it completed.
Check the box "View Whole", click on "Edit Tiles" then "Preview", and scroll over to where my polygon used to be. The overlay worked, kind of. It replaced all the tiles on the border of the polygon, and those only. The interior of the polygon remained the original tiles.
I was able to brute force replace them with overlay tiles, but I'm guessing there's some easy way to fix it. Maybe some option somewhere I should/should not have selected? Maybe there's a different way than outlined there? Any hints?
Second, I scanned through and maybe I missed it, but are you missing this tutorial? Do you want it?
Thanks