Phormium is fairly forgiving of root loss - it grows in distinct 'fan' arrangements in terms of foliage, and as long as you get some good root on the bottom of the sections, it should cope when its replanted if kept well watered. Unfortunately, though, the best time of year to do this is actually late spring, but you've started so you must now finish! If you can't get the whole thing out intact, cut through vertically, in between a fan arrangement, with a breadknife to section it a bit and then dig up the various clumps. I'll admit when I do this, I tend to pot up three or four sections into deep pots (like a rose is supplied in) and keep them somewhere sheltered and where I'll remember to water them, just in case the bits I've replanted in the garden don't make it.