I just upgraded my phone from the Nexus 4 to the Nexus 5. Unfortunately I lost a few numbers that I only realised after I formatted the old device. I’m sure I’ll be contacted by anyone missing within time.
I’m glad I sorted out Guild Wars 2 before I formatted, it is quite a hassle to remove the authenticator and requires the original 2 step, before I could add a new one.
The Nexus 5 supports 4G, however my current giffgaff goodybag is currently 3G. I’ve changed it so that the next goodybag will be a 4G package, however that is a month away. There is a way to cancel a goodybag, you need either less than £5 of credit, or less than 5MB remaining. I opted for the second one, and began to download a 500MB file, which will then be followed by another 500MB file until I have exhausted my goodybag.