cynical he wants to change the other way. it's all in the thread(ed/less) title

griffin yes you can change for sure. the thing is they are fundamentally different ways of doing things.

if you currently have a threadless set-up you will need to change fork (or at least steerer but since they are attached to the fork and replacing the steerer alone usually costs more than a new fork then assume a new fork), headset and stem.

this is no big deal. the only obstacle is if your bike has a 1 1/8" headset/steerer in which case it's gonna be a big PITA, as stated above.

if it's 1" then you simply need to purchase a new fork, headset and quill stem. these will take the place of the old parts and don't require anything specific framewise (ie either set-up will work on the same frame). you can then sell the other stuff on eBay.

with threaded forks headtube length is more important than with threadless, so check with someone knowledgeable that the steerer is the correct length, since it's not simply a case of cutting any old steerer to length as with threaded.

the rest is relatively simple. as long as you have all the correct parts, you can run either set-up with ease.

i myself have the lower apparatus from a threadless headset and the uppers from a threaded, because in both cases the lower pieces, being simple bearing races, perform the same function.

the upper parts are crucial, because they compress the races and make sure that the steering pieces are attached to the steered pieces.

anyway, simple answer is yes