I don’t really know why this is happening, I tried updating the same project from 24 to 25, then to 26, and then to 27. It just breaks when updating to 27, breaking meaning the editor user interface gets washed out. It seems the color theme is messed up for some reason, but I don’t find the way to change the color themes in Unreal 4. I am updating several projects from 4.24 to 4.27 and thus far just this one got this error.
SOLUTION. I ended up making a new empty 4.27 project and migrating the levels from the washed-out project to the new project. Not ideal but at least it solves the issue.