When I re-do mine (hopefully within the next few weeks), I'm doing "both"
-- existing centrifugal pump will remain as it is, wired through the battery switch and plumbed from the forward bilge pocket where it sits on top of the keel-bolts
-- new diaphragm pump will be wired directly to battery, with a fuse block on the battery post as Loren has done, and plumbed to the middle bilge-pocket with a water-witch switch as Christian has done.
Partly because that means I can get the new system wired/plumbed/tested before decommissioning the old. But also partly because keeping the old one doesn't hurt anything, so (for the moment), why not.
ObNote, that "should" mean that the centrifugal never activates because, if the new system works as expected, water will never get high enough to trigger the centrifugal's internal float-switch. We'll see. There's a "tee" fitting at the exit-thru-hull under the transom where the shower-sump plumbing used to connect, so I can easily run them both to that exit, but I do know that if there is ever a time that BOTH pumps are running, that single-exit will constrain the amount of dewatering that happens.
Bruce