Updating without a sim is no problem, it only needs your account and of course it has to be unlocked.
Sorry Sarge, not true. We have an unlimited data plan and my phone is the only one to use it. I tether and then the other phones and PCs in the home can access the Internet via that tether on my phone. I used to use a Moto X but it was so damned unreliable at tethering because it places internal work before the tether QoS, that I bought an unlocked 520.
The 520 is 100% reliable at tethering so I decided to update it to 8.1. I put my SIM back in the X and then hooked the 520 to the tether via wifi. I paid the $19 dev fee, I know it could be free, but it's only 20-bucks and msoft needs all the financial help it can get.
I started the update process, had the update-App sitting in the Apps list and fired it up. Finally it got to the point where the download would start and the 520 was told to call home. Oooops, no SIM. That was the end of that plan, however the good fairy was looking after us as there are several threads running on poor tethering performance of 8.1 so it is a good thing we are still using the 8.0 as the tethering hub.
Bottom line, the 520 needs a SIM for that call home. I suppose I could have tried swapping the SIM back and forth but could not be bothered