Noooooooooooooooooo!  No need to drain down anything except the rad itself.  Turn off both valves and drain the rad contents into a low container (then into adjacent bucket).  

To drain a rad you crack open the connecting nut between the rad and the valve (either valve) and when water starts to flow, open the air vent at the top of the rad as this speed up the process.

When the rad is empty you can remove it (leaving the valves on the pipes!)

The old rad will still have some cruddy water in it so be aware of this otherwise you'll leave a trail of black oily water across the carpets.

Anyway, install new rad (assuming it is identical size), open valves, bleed out the air and you're done.

Approx time for this should be less than 30 minutes.  

Ah, if you're changing the valves then you will need to drain down (or freeze the pipes as we do), but changing a rad for an identical size rad doesn't need the drain down if you keep the old valves.

Reasons to avoid a drain down are:

∼ The time it takes to drain down
∼ The time it takes to refill and bleed all the air out
∼ The cost of replacing the inhibitor
∼ Possibility of airlocks when refilling, especially if the installation is unknown to you.

These reasons are why we freeze the pipes as I reckon a drain down and refill costs me about £50 - £70 depending on the house  

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We use large c0² cylinders, this sort of size and the Rothenberger hoses and freeze heads.  Bit ott for diy use.  Not really worth hiring anything for a one-off as the cost will outweigh the possible benefits.  If you are going to hire then get an electric one, these are slower to freeze but keep going as long as you have electricity.

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If you're just changing one valve there's no need to freeze both supply pipes, just turn off the valve you're not freezing and empty  the rad in the normal way.  When the rad is removed you can check that the pipe is frozen by carefully opening the valve.

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Can be, I've run out of gas before while working in a hot airing cupboard.  Panic, me, no.....................    We now have two of the c0² cylinders so we always have a full one as a spare!  Using the electric freezers means you don't have this worry but the freeze heads on electric freezers are rather large whereas the gas freeze heads are tiny by comparison.  This can mean the difference between being able to freeze or not.