Increasing a mob's damage with a procedure

Okay, so I am looking for a way to make it so that when a mob reaches a certain amount of health, a variable or something activates that causes it's damage and other stats to increase. (I am aware that I could just replace the entity with another entity that has the attributes I want, but I would rather have a seamless transition rather than it being a jumpy transition to a completely new mob, where you can see that they're health goes back up to full). If anyone could offer any help, that would be great, thanks!