Up for six.
Shimano's Centerlock to 6-Bolt Disc Rotor Adapter is a pretty straightforward article. It slides onto the centerlock mount on your hubs, leaving an interface to mount a six-bolt rotor. It includes a six-bolt washer, a lockring, and a circle clip to secure the conversion, and a standard Shimano lockring tool is all you need to install it. It's a home mechanic's job, but note that each kit only adapts one centerlock hub, so you'll need two for an entire bike.