You can't really manipulate it in the way that you pick who leaves.
But yeah, that way you can easily get people their dreamies - but you can only offer one at a time unless you save before they go in boxes and then get a whole town full of them - but that'd require a lot of careful planning and TTing so this is easier.
Indeed, if you get a villager in boxes and make a back up on that day before he's adopted, then you can put that back up back and you can have him back in boxes (but he'll still be in boxes).