There is no T-Mobile 1520. Ignore the current carrier and focus on what model the 1520 is or the original carrier.
Just scroll down the news feed, I think the articles pertaining to the 1520's mass update is still recent.
If its an ATT model, I believe yes, its out.
I know they're is no TMO 1520. But how can I get an att update on TMO? You sure?
Check the news, my memory isn't infallible, but i am very sure ATT released 8.1 + Cyan for their 1520's.
Whether you're on TMo or some other carrier is irrelevant. An ATT phone with ATT branding will receive the update from ATT regardless of whether it currently has ATT service. You would not need to do anything other than either downgrade if you have the DP or continue to wait.