They're right: don't do it. Why would you want carbon forks if you have a perfectly good 531 set?
The other problem you may have is that your nice old 531 frame will have a 1" steerer. Carbon forks will mostly be built with 1 1/8" steerers - they just won't fit. Plus, unless the geometry of the carbon forks exactly matches the 531 ones (unlikely - fashions in frame/fork geometry change with time), you may have problems with toe overlap or strange handling.
I think your confusion over threaded headsets (which is understandable) stems (sorry) from having only had a good look at threadless ones. Take a close look at a threaded one sometime and all will become clear. The headset has nothing to do with adjusting handlebar height - it simply holds the steerer inside the head tube and allows it to swivel. Then the stem clamps inside the steerer and takes the job of holding the handlebars from there.
And the whole setup is SO much more elegant than a threadless arrangement... Threadless is fine on a modern alloy frame with big ugly blobby TIG welds, but totally out of place on an elegant hand-brazed lugged steel frame.
As for ease of servicing or dismantling, well, yes, perhaps it's slightly easier with threadless, and no special tools are needed. You'll need one large spanner for a threaded headset - you probably already have a plumber's wrench or Stilson or big adjustable spanner that will do it.
But headsets don't get a hard time - they don't do millions of revolutions under torque like bottom brackets or wheel bearings. So they don't need servicing very often. A headset like Stronglight's has caged roller bearings and replaceable races over alloy cones and nuts, so doesn't corrode and it's actually more serviceable (as opposed to, chuck it away and fit a new one) than most threadless bearing sets. Not that you'll ever wear one out anyway.
(Funny, it was threadless headsets and utter frustration with them that brought me to CycleChat, or Cycling Plus as it was then, way back 3500 posts ago in July 2007...)
