I have an AT&T Lumia 920 without a SIM at the moment. I like to hop phones with my SIM, so I can get to know other OSes. I currently have my backup TMobile SIM in a Z10 after switching from my Lumia a few months ago. As 8.1 seems to be a huge update, I'm considering switching it back in so I can familiarize myself with WP 8.1 as I did with 8.0. Will it let me know when the update is ready even though I don't have a SIM in it? It's an unlocked AT&T model that I use on TMobile.