It doesn't matter which carrier releases the version, it simply needs to support your hardware model (don't try to load 8830 software onto your 8820). Cellco support reps who say you will brick a compatible phone are simply following orders from above, and using fear to keep their task simple by only having one version to support.
Look at the top of the 8820 message board, you will see two sticky threads for the 4.5 OS with upgrade unstructions. (Critically, backup first, and remember to delete VENDOR.XML). Read the threads for user comments, I see several T-Mo users reporting success.
The only thing that matters is the model number. Find the 8820 and download it.
Ensure you are deleting the vendor file (although I never did, but just to avoid any issues.) I would utilize Pete6's app as it does all the work for you.