How to run large schema changes

Foo

Schema Migrations
   * Master for writes Passive master(slave) for reads
   * Kill all slave db connections.
   * Mark slave readonly.(safety)
   * Mark slave offline(for the application)
      * Should make the application transparently/otherwise switch reads to master
   * Run schema migration on slave. Have some coffee(hours)
   * Let Slave Catch up with the master.(some hours)
   * Bring the site down
   * Kill all master db connections.
   * 
   * Make slave the master.
   * Bring the site up.
   * Old master replicates from the new master(But still not online)
   * Mark new Slave Online
   * App now seamlessly/otherwise reads from slave