Re: So did Worf's Change of Heart cost him command of the Enterprise?
Technically, Decker never left the gate. I don't remember if Spock ever took the cadet crew on a training cruise, but when Kirk came aboard Spock insisted on relinquishing command. "First best destiny" or something.
For clarity's sake, Spock was in command of Enterprise during the training cruise and didn't relinquish command until after Kirk brought the Regula I situation to Spock's attention.
Funnily enough, had Spock remained in command, it's conceivable the shields would have been up during the first encounter with Khan
"You have been examined. Your ship must be destroyed. We make assumption you have a deity, or deities, or some such beliefs which comfort you. We therefore grant you ten Earth time periods known as minutes to make preparations."