Interesting that they happened to announce that today, what odd timing. So I downloaded the current beta for GMS2 and was able to open the project, edit files, ect. I exported everything in the project into a local package- but trying to import it into an LTS project makes the program hang infinitely. The reason this is unfortunate is because I was hoping to make use of this engine as a Mini-Game within my main project which uses LTS. I hope the update at the end of the month will make some magic happen somehow, until then I'll tinker around with it in the GMS2 BETA and just hope! I'm getting a feel for it and I really like how easy everything is to edit, as always, well done!
Glad you figured out SOME way of opening it at last, at least... in the worst case, it's always possible to manually copy code from the GM Beta and recreate objects in the main project (should be a lot easier with two instances of GM so you can actually compare stuff properly, vs doing it by hand from the .gml / .yy files)
(The one thing that's not super obvious is that I use the GUI "Variables" interface for all the enemy stats, they're not set up by code in the Create event)