If you are not trying to change any of the dimensions of your position, why would you go through this costly exercise? You will have to change the fork- the steerer tube of a threadless fork cannot be properly threaded for a traditional headset. All of the dimensions of the existing fork will have to be maintained for handling purposes, and then you will have to figure out the stack height of your new headset to make sure you get the proper length steerer on the new fork. The new stem will have to be the same extension as your existing threadless one, and then you need to make sure it is the proper diameter to match your existing handlebar, unless you plan to replace that too.