So since AT&T has apparently completely forgotten about the 920, I'm so very close to buying today's Amazon deal for the 520 so I can have 8.1. Just until the 920, if ever, gets the update. Worth the $40? Hmm..
Going from a 920 to a 520 just for cyan is not worth it. I bought a 520 as a stand in while my 920 was in for warranty work, and I hated it. Now my mother in laws has the 520. When the new phones are released this fall I will probably update and the 920 will be my "new" stand in. Just bide your time and wait for the official release. Probably not that much longer.
I went from a 520 to a 920 recently, massive upgrade, you would loose a lot of features downgrading...but the 520 is a great device for the cost, but I couldn't go back