If your CH system is open vented, you will need to either drain the system, which will involve turning off the cold water mains feed to the F & E tank in the loft, or you could 'bung the tank' but that is a more advanced operation and less advisable if the rad you want to change is on a floor level above another room or someone else's flat!
If you choose to drain the system you would need to drain it to a point as low as the pipes running horizontally under the rad you wish to change. If you choose to turn off the radiator valves of the other radiators to avoid draining and refilling them, there is a risk that one of the valves will 'let by' causing a dribble of water even after you think the pipes are drained. Or these valves start to leak once touched. If you drain all the rads you will need to open the bleed vents on all the rads, and rarely but sometimes the heads of the bleed screws are rounded and won't undo.
If you try and do it by just turning off the valves on the rad you want to change, assuming these valves turn off and on reliably, you still have to get the water out of that radiator. If there is enough free play on the pipes you can separate the valve from the valve tail that is screwed into the rad far enough to get a controlled stream of water into a tray, keeping a larger bucket on hand to empty the tray into . Often the water just dribbles down the valves, down the pipes and into the ceiling downstairs. You should tie a tea towel in a tight know around the bottom of the pipes and have lods more tiles there to catch stray water. An aquavac is also handy. If you are quick and strong you could loosen valves from tails on both sides simoultaneously and lift the rad off the brackets and turn it upside down before too much water has ****ed out onto the floor.
Once you have done that, you either have to find a radiator the same length, transfer the valve tails to the new rad using a special tool, or fit new tails that are the same size as the ones for the existing valves. There is a little adjustment in length between pipes can be made by the amount you screw in the valve tails, but not much.
If the new rad is a different length, you will need to re pipe, preferably from below floor level to make a neat job. If there is too much water in the pipes to allow soldering, you can use a push fit. With a de-pressurised sealed system, or a bunged tank, an experienced plumber can chop the pipes and attach push fits very quickly, remembering to clean the pipe before cutting.
And then there is the issue of the wall - bracket positioning for the new rad may well be different from the old, If it's a stud wall then you may need to run a baton or ply between studs to support brackets, whose positions are defined by the pipes if you are not moving the pipes...blah blah blah my point is that it is unlikely to be a blue sky scenario!!
