How to trigger a procedure after trading with a villager?

I'm creating a mod where you build an army, and part of this is Voluntary Soldiers and Conscripts. The basic idea is unemployed villagers can start working at a job block to become the Recruit Profession. From here, you initiate a trade with them, which upon completion turns them into Infantry, which is a separate mob (it would spawn the infantry and despawn the villager)

I got the job block, the recruit profession, villagers turn into the recruit profession, but I cant find out a way to get the villager to turn into the Infantry. 

Is there a way that I can make the trade itself turn them into another mob?