Not that simple. The top race on a threaded headset is (duh) threaded and too small in diameter for a threadless steerer to slip through so you, at a minimum, need the entire upper half of a threadless headset. The lower cup, bearings and crown race of the threaded headset could be reused but, since you probably can't buy just the top half, you might as well install both new races.

Yeas ago when threadless headsets and forks first appeared, Shimano sponsored pro teams wanted to use them but Shimano didn't (and still doesn't) make threadless headsets. The team mechanics machined out the threads of Shimano headset upper races to make them fit threadless steerers but this was precise work and a stop-gap at best. Don't do it.