I'm a fan of Windows Phone 7. Is it possible to downgrade a Windows Phone 8 device to Windows Phone 7? I got a Nokia Lumia 930.
This is not possible as Windows Phone 7 supported different processors.
This question was really unexpected!
Short answer: no.
Windows Phone 7's last update is dated back to 14 March 2013. Also, the last few updates didn't included new hardware support but only some software improvement. Without hardware support there is no chance to get WP7 working on a Nokia Lumia 930. It's not only about processor or instruction set: screen resolution, new and faster cellular hardware, new sensors on board, etc. Basically all new hardware that wasn't available at the time when Windows Phone 7 was in development. Since the release of Windows Phone 8 - that was nearly two years ago! - all the Nokia Lumia phones come equipped with the new version of the OS.
Frankly, doing such downgrade, even if possible, would be highly inadvisable. You would be unable to install lots of applications (even Microsoft has pulled the plug of WP7 and doesn't update Skype anymore) and you would missing hundreds of Windows Phone 8.1's new features.
If you really like WP7 (and retroware attract you: WP7 support ends today), have a look on the internet: you can surely get for a cheap price a WP7 device for playing around, but I firmly suggest to enjoy WP8.1 for your daily smartphone needs.
You need to make a .bsp file for it. The same as if you were installing windows ce 5.0 or 6.0. That way the OS knows how to deal with the hardware.. From there, you can build the image and (somehow) upload it to the phone. You have to have a copy of the build environment, or know someone who does.
I went thru the same crap with my Lumia. (unsure of the model because i paid 20 dollars for it from a friend and it just sits here with the battery out) I want either windows mobile 6.5.5 on it or even windows ce 6.0 CE6 would be the best because it's got a start menu and real windows not just that fullscreen crap. (I can't stand that fullscreen list view or the modern/metro lego block screen that all new windows OSs have. when I got the phone I asked "can I install classic shell on it? lol.")
Never was going to use mine for a phone or gps, I just wanted a cheap windows ce based PDA and was told it was possible. (I should have figured it wasn't probable tho) A windows CE image needs to be "built" for the specific device you are planning to use it on (or one similar enough to run decent and not brick)
If you don't care about everything working 100%, it might be possible. Especially if you can find a group of people who share the same desires to downgrade and have moderate windows CE building skills.
Android might be easier to get running since its open source and free. From there, you can probably emulate a windows 7 phone but it won't be running natively.
Give it a few years (like I'm doing), maybe when it's "old hardware" a scene might pop up with that specific goal in mind and the resources to pull it off... Or just find the closest model with phone 7 and try to reflash it (praying it don't brick along the way)