If the taproot is not hugely inconvenient, I wouldn't necessarily cut it all off, especially if that's where most of the new feeder roots are growing from. What I would do is cut part of it now, and maybe the rest next time you repot. It's just safer. Don't be TOO careful though, at least 1/3 or so can be taken, if not 1/2, because you need to encourage roots to grow from above in a wider area. Teas like high humidity (but from a wide tray of stones, or room machine rather than spraying), very high light (i.e. full spectrum fluorescents 5-6 Inches above it for 15 hours a day, and if your mix does have a lot of grit in it for fast drainage (a good thing), water when no more than the top 1/4" has dried, but not if you're trying to grow it in potting soil full of peat that never dries (at the roots anyhow) and can rot roots, though the tree will suffer as it should be in bonsai soil so it can be kept moist (NOT 'wet') throughout.