I changed the impeller on my 2002 25 merc about a month ago. Impeller cost $22. The shift rod disconnects near the carb. You have to remove the starter to get to it. Disconnect 5 bolts and the foot drops off. There is a washer on the shift rod that will probably fall off. I'm not sure what purpose it serves but be in a clean spot where you can find it. The motor needs to be in forward so the shift rod will connect back up. Took me about 1 1/2 hrs. About 1/2 hr spent on trying to figure out how to disconnect the shift rod.
I don't know much about the four strokes, but if it is a 2 stroke it should be a piece of cake. My brother has a 2000 25 merc and he can change the impeller in about thirty minutes. I've never done it myself, but he sure makes it look easy.