Flat bars suck.
A better option is just to install a set of interruptor / cross brakes and maybe some bar gels, and ride in the tops as much as you like. That's a cheap upgrade that pretty much gives you the flat bar position, with the option to use the hoods or drops.
But if you insist on flat bars, you'll have to do a fair amount of changes.
• You'll need new brake levers, and they'll have to be compatible with the existing canti brakes.
• You'll need new shifters. Finding compatible ones shouldn't be that hard.
• You'll need a good pair of grips and bar-ends as well.