I don't think it will be that simple. Compare the top tube length of a road bike(drop bar) and a hybrid(flat bar) and there is usually a difference by 4-6cm. If you convert a road bike to flat bar you will need a REALLY LONG stem to the reach that will optimize your position. You can compromise the bike's handling by doing this since there will be too much weight on the front end of the bike. You can convert road bikes to bullhorns since the reach is about the same if not slightly more than drop bars, but you really have to consider getting a bike frame that is generally longer on the top tube when you are considering putting flat bars on it.

Converserly, the same thing is true when trying to convert a MTB to drop bars. The top tube is way long so you need a really short stem and once you do this the handling is affected in a negative manner that the bike was never designed for, there will be too much weight on the back.

If the OP wants to convert a road bike that originally took drop bars, he/she would be better served by getting a bike one size bigger in order to get a longer top tube or just buy a performance hybrid like the Cannondale Quick that is already designed for flat bars.