If you're uncertain the impeller has been changed in the last couple of years, just do it. It'll give you peace of mind and and is a lot cheaper than buying a new powerhead.

Look through these yamaha topics: http://www.outboardrepairs.com/yamaha/

...might also read some of the threads in the Yamaha forum here: http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi


good luck--