Changing Battle background image Mid-Battle

A friend and I are working on a Pokemon fan game that has to do with spirits and illusions. It's a dark spin on the traditional Pokemon games, but that's beside the point. What I'm here to ask is this:

How can I change the Battle Background mid-battle?

The reason I ask this is because (spoilers to follow):


Thanks in advance to anyone who can help us. :)

As a backup idea. You could simply make 2 versions of the Gym Leader. And have the last version use only the one pokemon. By doing this, you should be able to use different music and background. Hope this helps a bit.