The gap should be fine as it's only 10mm you'll probably be able to pull the pipes; or get yourself a pair of rad tail extensions.
Simply turn off both the valves on the rad you will be removing; use a flat tray like a decorators tray, or my fav a cat litter tray (They hold more water) and place it under one of the valves; undo the nut and allow the water to drain out; when it stops or slows to a trickle; open up the bleed point to get rid of the rest of the water in the rad.
Once emptied; close the bleed valve, undo both the nuts fully... Lift the rad off of it's brackets and flip it upside down; remove the two tail pieces (Which will now be at the top and are the bits where the nuts will be connected onto) carry it outside and dispose of it at a metal recyclers - Not in the dustman's truck; this is just wasteful....
Unpack your new rad; fit it's accessories; and install your old tails into the new rad; try and keep them at the same ends as they used to be to ensure a nice seal; use plenty of multi-wrap (White cassette) PTFE on the threads; I find the single wrap (Yellow cassette) doesn't sit as nicely in the threads...
Hang the new rad on it's brackets (You may need to change the brackets; you may not if you're very lucky)... Connect the rad valves onto the new rad and tighten them - Do not use excessive pressure; it's just not needed).... Open up one rad valve to allow water to enter the radiator; check for leaks and adjust as needed... Then open the bleed point and vent the rad...
Open up the other end and return them both to their original positions (So you don't have to re-balance... Minor adjustments may still be required as this is a larger rad)...
Check the rad is heating up nicely - You may need to top up your boiler as it is a combi
