Sorry mate, are you talking about stripping down the old leg to get the top mount off to use with the new one. Sorry i don't follow what your saying.
Basically, you need compressors on the spring when removing the top mount (if you are) from the old leg and re-fitting. If you undo the top mount with out the compressor the likelihood is you'll die when the spring goes off in your face.
But once you've got the new leg built (i.e. top mount is securing the spring to the leg) and your ready to fit to the car you no longer need compressors.
Be aware - if you are stripping down the old leg 318iS springs are long and they're a bitch to compress fully. Even when compressed if your undoing the top mount there will still be some tension in the spring so keep your head out of the way.