I just purchased a 2016 MacBook Pro but have a 2012 MBP Retina I use for work. I'm considering buying the LG Ultrafine 5k and I'm wondering, will I be able to use the display with my 2012 MBP? I don't mind if I don't get full resolution while using the 2012, I just want to know if it will work.
Firstly, the Ultrafine 5k only has thunderbolt 3 over USB-C inputs, so you'll need a thunderbolt 2 to USB-C cable to even connect. You should be able to run the monitor at a greatly reduced resolution which is then "upscaled" to 5k. For instance, many people run 4K monitors at 1080p, so the 5k should work at 1080p (or other resolutions <5K) as well.
Upscaling is very common on TVs at the moment, since there is a lack of 4k content, and thus those watching 1080p content on 4k TVs must have their content upscaled. I've attached an article on C|Net discussing upscaling.
Thus in principle it should work. I'd contact the manufacturer directly to make sure. That monitor is not cheap.
Officially the 2012 MBP isn't going to be able to use the LG UltraFine 5K. For more information you can read Use the LG UltraFine 5K Display with your Mac.
Now, sometimes what Apple officially supports isn't the full story, so it may be you could find a way to get this to work. Only you can decide if that's a risk worth taking.