Threaded to threadless headset/fork?

Considerations:
(assuming you are dealing with 1" as most road threaded stuff is)

• not all 1" headsets are the same, there are 30.0 and 30.2 (check your headtube or existing headset)
  
• 1" forks are becoming hard to get, consider is it worth the upgrade V just getting a modern 1 1/8" frame and fork?