I havent found clear explanation how do make units switch sides.
That's because it can't be done.
If you want to make Resistance soldiers fight Resistance soldiers you can fake it. Create a group of Resistance soldiers under a West group leader. Set the probability of presence slider of the leader to 0. Do the same with another group, except of course make the group leader East. The two groups will now shoot at each other.
If you want them not to shoot at each other for part of the mission use setCaptive true/false commands.
Oh, and
Welcome to the forum!
I commend the forum Search function to you. (There's a button at the top of this page.) I answer this question most days: it has replaced "how do I get a fire to light?" as the most frequently asked question, though it does come in many slightly different forms.