Presumably both of these phones will be receiving the update in its entirety, personalized lockscreens and all. Unfortunately T-Mobile did not mention when exactly they will be pushing the update out for its customers, but they did give a rough timeframe of “this summer” so that’s something we can look forward to.
It’s actually a good thing that T-Mobile and Microsoft aren’t neglecting its lower-end handsets when it comes to the update. Previously Microsoft made that mistake with Windows Phone 8 where they basically neglected their entire range of Windows Phone 7 handsets claiming that hardware was one of the reasons they could not upgrade it to Windows Phone 8.
We can only assume that if the Lumia 521 is able to receive the Windows Phone 8.1 update, the Lumia 520 should not have any issues either. Either way hopefully more details will emerge later on regarding the update and compatible devices, so stay tuned!