I agree that it is poor engineering and that is why they have changed that pump to one with a front opening impeller cover on the very latest engines - I was told that they have received more complaints on that one feature than any other. But, to answer your question: yes, you do have to take the pump off the engine to change the impeller - at least that is quickest way - I can now have the pump off in about five minutes and the new impeller in and reinstalled on the engine in about 20 minutes (but I have had a lot of practice ).