Yip, no need to drain the tank, but while you are doing this you might as well take full advantage of this and give your rocks a bit of a clean, get a bucket and fill it with salt water, when you pick up a rock, dunk it in the bucket and give it a good swish to remove all the loose detritus in the rock.
You might also want to temporarily make use of some filter floss as you are going to stir up quite a bit of muck in the tank, the floss will help to catch a lot of the muck, whatever else gets through will hopefully end up in the skimmer.