It is not possible to re-allocate plants.
It is technically possible in the customizing client, otherwise you could never correct a mistake when you initially setup your enterprise structure.
But if your plants were productive, then you cannot just switch a plant from one company to another.
this customizing switch will certainly not trigger thousands or even millions of background transactions to move the stock values from company A to company B, to move G/L account values from A to B etc etc. This is a dream only. if you do this switch in customizing, then you are just creating an inconsistent system, nothing more.