Leave the hose in place and swap the callipers and levers over.  Bleed.  Job done.

Thought about that but want to sell the XT brakes so don't want to give them old hoses.

Tape the new hose to the old one, or failing that, at least attach a piece of string so you can pull the new hose through.

Get a double ended barb like the reverb one, attach to old and new hoses and pull the old one out and the new one in at the same time.

Due to the banjo at the calliper end, I doubt the techniques above will work.

If you remove the olive and barb, you should be able to insert the end of a gear cable into it, and secure with a wrap of tape.

Pull hose out of frame from banjo end, making sure that the nipple end of the gear cable doesn’t disappear inside the frame - you could thread a washer onto the cable beforehand to prevent that.

Swap hoses on the cable, re-tape and pull it through from the other end of the cable.

Should work?

I'll confess to dropping down to my mate int he bike shop and getting him to do it 🙂 Much simpler as it needed to be shortened and bled too

I'm tempted to ask the bike shop to do it, but I like learning these things.