Actually due to the low cost of hiring, I’ll just fire and replace them. Just rename them to be exactly the same name. Think and imagine of combat school for RPG purposes if that breaks the immersion. Or have the main character telling the guys “Get Gud!” and paying for their combat training.
Trying to level up to the same level as you main just invite so much more pain due to the initial starting XP disparity you would already have.
Although it may not work on special classes like the Prototype though. I’m not sure if you can rehire the special classes you get.