Changing an impeller is not a daunting task and it's one that every boater should consider adding to their arsenal of DIY maintenance. Changing and impeller, on many engines, usually takes less than 20 minutes even if you are fairly slow. The most complicated aspect of it is access to the pump.
Impellers are a wear item unfortunately they do not always abide by a "time" or "hours of use" schedule. A simple mishap like forgetting to open the engines raw water intake can kill an impeller in short order. They don't like to be run dry and they don't like heat. If let go for to long they can "throw" a blade or loose a chunk that can lodge in the hose or the heat exchanger. If this happens you have a much larger job on your hands of finding the piece to prevent further damage or overheating issues.
I change my impeller every year regardless of condition. Some feel this is overkill and you'll need to decide for your self what you maintenance schedule is. I look at it this way, impellers are cheap insurance! I pay $16.00 for the impeller used in my Westerbeke diesel engine and it takes about 8-10 minutes to change it. It took me twenty minutes even while photographing it and the access for my large camera was very tough!
An over heated engine or a thrown blade can cost many, many times more in both time and money than the few minutes and the "low" boat bucks required of this simple maintenance. Heck, changing an impeller costs less than changing my engine oil and filter.. If you do it every spring, or every time you run it dry for more than a 30-45 second period, or under restricted flow due to a blockage in the strainer or intake system you should have a trouble free impeller and never throw a blade or lose flow due to a worn impeller again.