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But you can
Well, yes, you can if you can be arsed to make copies of the factory assembly tools, but the freehub unit is always sold complete so even if you could get it apart you can't get replacements for any of the wearing parts. Compare that with most boutique hubs which, for reasons of manufacturing economy rather than design excellence, use a couple of cheap industrial cartridge bearings to spin the rotor, and insert the ratchet into the hub shell.
But you can, best thing about Shimano is the cup and cone system.