you can swap their place! by clicking on the tomb and clicking on another in the shop phase.
Also, the freeze is so the unit in the shop stays there after rerolling or after a turn!
anyways, I am planning on making a few of these things about more clear, hahaha, as yeah, they can be quite tricky, especially because some of the mechanics are very gente related (games like super auto pets, auto battlers in general)
thanks!!! (: