The changes to jobs will be available to you on the 28th even if you didn't order Shadowbringers, but you'll be unable to level above 70 and therefore not be able to access actions and abilities for levels 71-80.

You won't be able to start a Gunbreaker or Dancer because the quests won't be available to you. You won't be able to progress to the new area because you will fail the check. Until you order and enter the code.