Threaded/threadless: same HT?
 

Is there a difference in headtubes for threaded VS nonthreaded headsets and steerer tubes of the same size? Can either system be swapped for the other in the same frame?

As long as the size (usually 1" or 1-1/8") is the same and the headtube takes press-in headset cups, you can fit the frame with either a threaded or threadless headset at will and use the matching type of fork.

Thank you.