Re: Ok to remove Gearcase Cover?
I am not overly familiar with your motor, but I do not see why you would want to remove your gearcase cover, just to change the impeller. It looks to me that you are pretty much ready to slide the new impeller right down onto that plate. Before that, I would inspect the plate for scratches and change it if necessary.
The warning I am more familliar with is when I am changing the gear oil, NOT to remove to phillips head screw in my gearcase (instead of the fill and vent screws for the oil). Doing this will dislodge the shift rod and an entire rebuild of the gearcase will be required to fix it. You really do not want to do that.
Those are kind of two seperate issues, changing an impeller and changing the gear oil.
Anyway, inspect that plate and the inside of the housing for scratches. If OK, slide that new impeller down the shaft and move on, would be my thoughts. Good luck