We used to say...
Yo Eddy! said:
From fattony's post I assumed he already understood steerers do differ
On making new threads in in a shortened steerer: I had it done multiple times. One bikeshop managed it to use a thread refresshing tool for the purpose. That was not so good action. They had to buy me a new -in this case- Miyata FatMax
...in the bike shop that it was OK to provide SOME new threads, but not ALL new threads. If your top nut is on new threads, you need to really trust the tools and the person who did the work. In other words, bike shop cutting tools aren't always the sharpest and bike store employees aren't always the best trained in how to use them correctly and we recognized our limitations.
That, however, was in the late 80's early 90's when no bike shop - decent or not - was without a full Campy tool kit and no bike mechanic was without at least a basic understand of cutting and facing. Things have changed.
If you're really considering doing this, on a very hard to replace fork, you should probably take it to a frame builder. Or go to the oldest, wisest bike shop in the area and hand it over to their lifer.
Or go threadless. Problem solved.