Don't get a bracket extension. Just purchase the correct adapter.
What fork do you have? Some forks are not rated for 203mm rotors. The braking forces can cause accelerated bushing wear, it can even cause the dropouts to crack. Make sure that your fork can take it.
Why do you want to move up to 203mm rotors? Are you experiencing significant fade? If not, there is no real good reason to. Bigger rotors are, of course, heavier, and they hang closer to the ground, and are thus more prone to damage.