Just changed the impeller on my 40hp - 2005 Yamaha. I did some research and posted on the outboard forum that I found. Then I bought a manual from Walmart. Manual did help but talking to the guys at the marina probably helped the most.
Hardest part was putting the lower unit back in place and getting it all lined up. I did put the motor in forward gear before I dropped the lower unit. The Marina and several said to do this.
Manual was a Sealoc
Purchased Yamaha OEM Impeller, I only changed the impeller. Everything looked good so just changed impeller.