Assume it is a 1" stem. You could do this as a partial upgrade: get a quill stem adapter, Profile Design among others, and then you can go to a more modern stem setup. If you want to go to threadless consider that you will need a new headset, fork, stem and maybe bars. Also if you do not have the tools you will have to get your LBS to press in the new headset. Is it worth the cost of upgrading? Try looking for some older carbon threaded stems on ebay as another option.