Yes, you can remove the existing user from the list, and add someone new. The old employee won’t be able to use SketchUp by signing in anymore, unlike with Classic, where if they made a note of the license they could add it to a personal machine after leaving you.
Something to know, there is a 60% discount offer, where you could get the first two years for $240 each instead of $598 each. I think you have two 2017 licenses, and should be able to get the offer on each one. Might be worth phoning sales to make sure you get the right discount.