Do what Onganju said... take the boxing. You'll get used to the new style and get used to footwork and shadow boxing. And the transition won't be too hard as long as you remember that you can't rely on just boxing in MMA. Practice your Burpees, stretching. And since you've take karate so long you know all the kicks and all so you should do fine and be solid.
You are blessed by the hand of your free chosen god