New derailleur, does that mean…

…new chain?

moving from an SLX 12sp to XT 12sp derailleur, despite current chain, cassette and chainring being all good (less than 500 miles and no concerning wear) should I be swapping any other bits to ease any possible issues?

Nah. All good.

Perfick! Ta very much

New cable/outer possibly.

Cables and outers will be replaced, have them on order with the derailleur

You’ll be needing to replace the rest of the drivetrain and might as well do the brakes at the same time. I’d suggest a new frame and wheel set to overcome any compatibility issues and obviously new bars, stem post and saddle will be required.

Your old pedals should be fine though

Your old pedals should be fine though

Axle rebuild and new cleats though. And shoes if the cleat bolts have seized (they have).

Suspension could do with a revamp as well!

Eugh…you can’t reuse that dropper!

At 500 miles I probably wouldn’t bother with cables. Don’t even need to chop the end cap off.

Is it just me or does it seem rude to put a new XT mech on old cables?

Just you. 500miles is not old unless the OP has been aiming a jet wash at the ends. Literally no need to swap them (or the chain or cassette) unless they are having shifting issues.

After 500 miles the outer cable strands will probably be sticking out a few mm past the outer cables outer layer, I like to trim that off but usually just put some new cable on when fitting a new part, most peeps don’t have a workshop reel of sp41 though.