Yeah maybe I wasn’t entirely clear in amongst my rambling. Procedure would be:
Front brake - unbolt and swap over. Easy.
Rear brake - find/salvage/buy a couple of metres of old brake hose, plus your new brake. Use hose cutter to cut existing brake hose close to lever. Use barb tool (linked below) to screw freshly cut end to bit of old hose you got. Now unbolt Caliper and pull old brake out of frame, drawing scrap hose into frame as you go.Bit of push/pull helps here. You now have the old scrap hose through the frame.
Now you need to cut last few cm of new brake hose off at lever end to get rid of insert (if already installed, I replaced with Shimano so it was). Now screw freshly cut end to the lower end of the hose that’s in the frame at the Caliper end, and push/pull the new brake up through the frame and through all the relevant grommets.
Once done, fit new insert (bought cheap hose insert tool off Amazon, made this a breeze), swap levers and do a little bleed to get rid of the air from the hose at the lever end. The precise details here will vary according to manufacturer but with my new SLX I didn’t need to do a full bleed, just used the little pot screwed to the lever to add a little oil and get the air out.
Bit of a ramble but hopefully it makes sense, shout me if not. Here’s the magic tool:
Barb tool