Disc brakes require not just special hubs, but also disc tabs on the frame and fork. I think some disc brakes can be mounted to the rear without disc tabs using a mounting bracket if needed.
If you get the headset replaced to mount a new fork, be aware that your bike will use a 1" steerer tube. New bikes for the most part use 1 1/8". Make sure you buy the right one!
Get a mechanic at your LBS to switch the headsets. This is a job that is easy to stuff up, and can result in a ruined frame and fork.
Is this a mountain bike? If so, I don't think there are many suspension forks made with 1" steerer tubes. If it's a road bike, disc brakes are rarely used...
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