how is early access going to work?

[White_Wolf_X]

are all of the combat/job changes going live on the 28th, but inaccessible to people who did not preorder?

All who pre-order shadowbringers and claim the early access key will be able to play shadowbringers content on the 28th, as the new classes are part of shadowbringers you very likely wont be able to unlock them until you buy the expansion.