Can you add something like barracks. They would be used to stall the knights current unit (upto a size), and later let the unit less knight to again take control of them. It would be really helpful to those who are wanting to lead a particular type of unit, say SF, but don't want to abandon his/her current unit. As for payment issues, like militia are paid from region income, the stalled unit is paid from the knights income.
Possible Exploits: What if the knight's income is less than unit payment? And what will happen if the knight leaves his estate (or kicked out) with his unit still kept in barracks?