As far as I could figure out, you should be able to choose the theater/seats from the opening day (March 30th) or some days before (probably once the theater puts it on the active schedule).
I'll be in Tokyo from 28th March too and planning to go see the movie, just need to decide whether to go to Kawasaki or Fushimi since the Hibiya theater link is not active (note says "Opens March 29th" and I suspect it will fill up fast). Probably Kawasaki, it seems closer. I've purchased two tickets so if you want you can join me :)
Also they will perform in the beginning of the first movie screening? Waaaah I want to go so badly. Why can't I have enough money to go to Japan? Grr. But yes, please spoil it. I want to know if there will be another album soon, or if we have to wait until Yuki makes a decision with them. *sigh* This is the worst kind of waiting.