I have a bike with a rear wheel with an M675 disc hub arriving imminently.

I want to put that wheel on another bike, with a 142x12mm back end.

Shimano make the M678 SLX hub, which to all intents and purposes, looks the same as the M675 rear hub but with a 142×12 compatibility.

My question is, can I buy the M678 hub, take it apart, take the M675 hub apart, and swap axles over? Or have I got to take the wheel apart and rebuild with a different hub (which will obviously be much more long winded)?

Without knowing the exact combination, most Shimano hubs are not compatible. I was unable to use an XT front hub that started life as q/r with 15mm axle even though there was a 15 mm option available in the same group set.
You could be lucky though!

Worth a try – I know it’s completely different, but I have currently got a 2012 XT freehub bodged to a 1994 STX SE hub body (to get 9 speed cassette on it – as it came with a 7 speed freehub) – it works well and didn’t need too much modification – both were QR type hubs but I would give it a go if I were you!

shimano techdocs should provide the answer no problem with exploded diagrams.

FH-M675 Diagram
FH-M678 Diagram

The two hubs had different freehub bodies. I assume this is because you can’t fit the larger axle through the normal body.