Rulers should not have the rights to move anyone around wherever they want. Lords have authority over their knights. If the ruler wants them moved, they have to work with the lord to get it done.
The duke idea as well won't fly. Nobles who don't have an estate are not aligned to a duchy, so no particular duke has authority over then. And if they do have an estate, then it is the lord's prerogative to assign to an army. Rulers are not absolute dictators.
