You can take rads off the wall without draining the system..................

However if something goes wrong you are fooked big time!

Personally I would spend the extra time draining to the system FULLY and then remove the rad.

If you are going to have the rad off the wall for some time and need the central heating on in the mean time then place some stoppers on the rads pipes (push fit plastic item are nice and easy) and refill system, bleed all rads. Of course you will need to drain the system again to refit the rad.