With the controller, you can have something targeted and then press left and right on the d-pad, it'll have a big circle (not on the ground) targeting an enemy B but you'll still be targeting your original target A. While this circle is up on B, you press an attack and that attack will activate on B and the circle sub-target will disappear and you'll be back to targeting A. Basic example: Target A, fast blade, savage blade, tap left, halone on B, target still on A, fast blade, savage blade, tap right, halone on C. It's controller's way of tab tanking as far as I know.

To permanently switch targets, you just sub-target someone and then press confirm and the target will change for real. Now the issue is that, when you sub target, the enmity list blinks, which would make you think that it would show enmity for sub-target but it doesn't; it still shows the enmity list of your main target. So for a controller tank to confirm enmity on any target other than his main target, he would have to press left or right along the d-pad and hit confirm and wait for the enmity list to refresh, essentially changing his main target.

Keyboard and mouse can select targets quickly so this might not be an issue and seem alien.