Great question — answering as the UphillTech team:
If you replace the frame, fork, wheels, groupset (inc. brakes), tyres and handlebars, you will have a road bike. You can reuse your existing saddle, seat post(if correctly sized) and pedals. This assumes that you want a proper road bike rather than changing the tyres, fork and gear ratios to make a mountain bike a bit more suitable for road-only use. Because that would still be a mountain bike. The differences in frame design, geometry, philosophy etc are explored elsewhere in other questions. It comes up quite alot!
Full answer by Noise: https://bicycles.stackexchange.com/a/96418 (CC BY-SA 4.0).
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