The Troop Academy is a newly released building that allows you to convert some of your existing troops into different troop tiers and variety. The building requires a troop academy Blueprint item in order to unlock it which can be found in certain in game purchases.
Once unlocked the building has 8 levels and each level requires Golden Badges to upgrade (found in packs). With each upgraded level the troop queue increases as well as the troop tier(s) that can be converted. Below is a table showing the benefits of each upgrade level:
In order to convert tier 3 troops into tier 4 troops for example you must have first researched and unlocked the troop tier and type that you would like to convert to. Once you have the required troops unlocked and the building leveled up sufficiently you choose the troop you want to convert and the troop you want to convert to and click to promote them, this then gives a timer which can be sped up as if normally training troops. There is item boosts you can acquire that will help to speed up the time taken to promote troops.
The Troop Academy is a very useful way to promote lower tier troops in especially as the game advances and the need for tier 1 troops in particular diminishes. The cost of getting the Troop Academy to 8 is relatively expensive currently but the increased troop queue that comes with the upgrade makes the process much faster.