Not possible - you need to turn it out of the pots and cut through the roots - if the rootball is large, cut it in half and pot the halves up separately.

I would do that in spring, myself. With some of the thicker-stemmed types--at least 3 1/2 cm thick--you can take a piece of year-old cane about 3-4 joints long, cut it just below the bottom-most joint, ream out the walls between the segments, except for the bottom one, with an iron rod, fill the stem with water, bury the bottom 5-10 cm in a gritty potting compost. Keep the stem full of water, and after a few weeks, leaves will grow from the upper joints, and a few weeks after that, roots and little shoots will grow from the buried joint. It works better on some species than on others.