No. You only pay the fee for using the office space for the first time. Think of it as paying for the computer and cubical walls for this new hier.
You can only replace a worker with an upgraded worker of the same type. There is no extra cost for this. Think of it as the same worker just getting better with experience.
You can have more than one worker of the same type in your office.
I am now going to incorporat your reasoning into my game explanation from now on.