Hi David,
Boy, this is an issue we seem to be seeing more and more on these later model iPhones. You're far from the first that this has happened to; in the past it usually happened when someone tried to remove the screen without taking out the Pentalobe screws first, but lately it's been happening regardless.
Anyway, you're still in the same position you were when you started; you've still got to get the screen off. Since the mounting plate that's supposed to stay on the display has come off, you really just need to repeat the screen opening procedure, only this time you're operating on the screen frame instead of the complete display assembly. Naturally a suction cup isn't going to help you at this point, so my best suggestion would be to use a curved X-Acto knife to get in between the screen frame and the phone's midframe and use that to pry up and create an opening. Similarly a thin metal pry tool like the [product|IF145-130|iSesamo Opening Tool] would work well; it just doesn't have quite as fine an edge as an X-Acto knife.
On some screen frames you can even pry up along the edge from the inside of the frame, but yours looks like it covers the entire display with no opening from the inside, so I suspect that's not going to help in your case.
Anyway, that's the goal, to get one edge up far enough to create an opening that you can get one of your opening picks into and slide along to split the adhesive. Once you've got that, the procedure is the same as it would be for an intact screen; go around the entire perimeter and separate the frame.
The good thing about your situation is you don't have to be quite as careful with the frame since you're replacing it anyway; otherwise you'd have to do your best to keep from bending and distorting the frame since it doesn't have the screen attached to it to help provide rigidity.
So basically that's it, outside of what you need to do to create that initial opening, otherwise you're just repeating the screen removal procedure. Heat is again your friend to soften the adhesive and make it easier to separate.
I hope this gives you the information you need; feel free to ask if you have more questions. Good luck, and let us know how it turns out for you!