I think this would be one of the biggest priority in the jcook builder UI.
It's really annoying for me, to remove a field because I have to change it from "text" to "textarea" and THEN have to rebuild all the places where that field was. (because when you delete a field automatically it is removed from all the layouts it was).

Done since 2.9.5
www.j-cook.pro/index.php/docs/versions/251-2-9-5