Hello Chrisby, sorry for the terribly slow answer. I did not get a notification for some reason. A working 4.12 version is now available in the marketplace, so this should no longer be an issue.
For future reference, though, if you want to open a project made in an older version of UE4 in a newer version you simply launch the newest version and open the old project from the window that appears. You will be prompted to open a copy, which will create a copy of your project for the newest UE4 version.