Threading is easy when you have the skills and tools, whether it's extending an existing thread or starting a new one. I for one would prefer to start new from a blank, rather than extend for a variety of reasons, but either way is fine.

There are a few issues, one is chrome which usually doesn't extend up to the top of the steerer, but can be managed if it does. The second and more problematic is if the keyway (if any) extends to the bottom or beyond the existing thread. Keyways are put in last because threading an interrupted cut always causes problems, so if the keyway extends too far a smart mechanic may decline the job.

The last one is purely financial, and one has to understand the problem facing the shop or mechanic. The cost of doing the job is fairly low in comparison to the value of the fork, so there's the question of who eats the loss if something goes wrong (as is easily can) and damages the fork. In industry standard contracts for things like heat treating where one may be paying $50.00 to service a part worth $5,000, limit the liability to the cost of the service.

No such standard policies exist in the Bike biz, and I've seen jobs like this lead to bitter hard feelings on both sides when things go wrong, so make sure the shop is qualified, and reach an understanding in advance about who eats the loss.