First, you'll want to remove your employee off the business pass. Once they are on the pass, they can login to aircanada.com and book using the pass without your permission. You do not have to book on your employees behalf.

Since the Sun Pass is considered "personal" and not "business", I'd doubt Air Canada is going to allow you to change the 2nd person, but you could always call and give it a shot. I would think by allowing you to change the second passholder they are opening things up for people to abuse the pass system (I'm not saying you are, but since it's a personal pass they have no idea if this person is your employee or your significant other.)

Good luck!