Mountain Biking - Changing Handlebars?

At the moment my bike just has the standard 'T-Shaped' handlebars. I was looking to get road ones that have the drops.

However, I assume that changing handlebars is not just as easy as changing a saddle, for instance, as the handlebar is connected to the gears and the brakes.

Also, at the moment, I change gears by twisting the gear controls on the handlebar (if you know what I mean). Nearly every drop-bar handlebars I've seen have no gear controls on the handlebar, instead on the top tube or something. Would this make the change more difficult?

Is it possible to change the handlebars without too much hassle? Thanks

you can change bars but its a big hassle, unless you are running oversized bars the clamp area is different so you need a new stem

modern roadbikes have shifters integrated to the brake levers, and these are very expensive, the brake lever ratio is also different so you need cantilever brakes or road ratio discs if you run discs, if you don't want to spend as much you can do bar end shifters, and non-shifting brake levers (tektro makes some that i like)

so you may be lookin at new bars, stem, brake, brake levers, shifters. thats not gonna be easy or cheep

There have been plenty of threads about changing flat bar bikes into drop bar bikes. Most of the time it's not worth the money though, it's usually cheaper and significantly easier just to buy a new road bike.