Change existing migrations to use new helper methods

* We usually shouldn't touch existing migrations,
  they would have already ran for most (if not all) users,
  and don't run for existing users since they start at
  the latest version.
* However, we've recently discovered many issues with these
  migrations, and past reported issues are now starting to make
  sense (they can be explained by migration running over and over again)
* As such, let's fix this once and for all to make things clearer.
* We also tend to copy older migrations when writing soemthing new so it
  should help having these done :)

Change-Id: Ic45efc823da7ea8a137e79b3769a9ca53c2f8667