Originally Posted by
Crankycrank
Depending on how much you're going to cut you may not have to lower the star nut but if you do all you need to do is tap it down further, no removal necessary.
Agree with this.
In the unlikely event the new star nut position would have to be above the old one, I don't believe one can actually pull it out to make it shallower. It's designed not to pull out, but I think the usual method is to tap it down out of the way, and then install a new star nut above it. Or use a longer bolt. But if you're shortening the steerer tube, like quoted above, I doubt that will be an issue.
And I want to add that cutting a steer tube is really an easy DIY, not need to get a shop involved if you know how to use a hack saw. It doesn't even have to be a full-size, high quality hack saw. Just beware you need to cut it accurately and
not too short - do a "dry" or practice assembly first to make sure, even if you "know" it's exactly a 10mm (or whatever) change.
It needs to be cut as square as possible. I put masking tape on it, and mark a full circle around the tube, then just cut slowly and carefully, adjusting to stay on the line all around the tube. I've also used a miter box I have which sort of accommodates a hack saw, but found that just doing it carefully by hand works fine. You can also use a file to square up the cut if there's minor high spots. But really, "perfectly square" is good, but not essential. The cut surface just needs to transmit pressure from the top cap, through the stem and spacers to the star nut to temporarily preload the bearings until you tighten the stem. It doesn't need to be perfect to do that.
Carbon or metal, I then take a flat file and dress the burrs and sharp edges off the outside edge of the cut, and use a round file to do the same on the inside edge.