You can change pitch yourself. Go to Harbor Freight and get one of the larger adjustable wrenches. You'll have to rig up a way to hold the impeller vertical. I think used a bolt through the impeller, held in place by a vice. You will also need impeller pitch gauges. I found a jpg image of them online and had them printed out at a local office max. Use their paper cutter to cut them precisely. Once you have the impeller in the vice, get an initial reading using the pitch gauge. Then clamp the adjustable wrench to the outside of the leading edge and bend down to lower the pitch. Make small changes and check with the gauge often.
There are YouTube videos and forum posts about doing this. I believe I have some pics of this when I had to do it for my boat motor swap (link below).