It’s a building called Troop Academy. This building let you promote soldiers to a higher Tier.
The process works by first choosing the soldier type you want to promote, then you choose the soldier type you want the soldier to be promoted to.
The question is – is it cheaper to promote a soldier or is it better to dismiss him and train a new one?
The answer is very clear. It’s cheaper to dismiss a soldier of a lower Tier and train a new soldier of a higher Tier rather than promote him.
The table shows a Tier 5 promoted to T6 Immortal vs. training a new Immortal
As we can see the cost of training a new soldier is almost 50% cheaper.
So the advice is – Dismiss soldiers and train new ones.