It should not be necessary to discharge the cap since it can be moved out of the way by unscrewing the bracket that holds it in place for the wrench to fit easily. Sometimes you can fit in the wrench without removing the cap but it only takes a few seconds and it makes the process so much easier.

HOWEVER I would disconnect the wires and plumbing from the pump and do the work on a workbench and not in the installed location! Trust me on this. Even with the breakers off it is still safer to disconnect the power wires and having the pump on a workbench will make your job MUCH easier! Remember that the impeller is reverse threaded so yo want to turn it counterclockwise (the opposite of righty-tighty, lefty-loosey).