how is early access going to work?

[Dustytome]

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

All job changes will take effect for everyone. Any skills/content beyond lvl 70/stormblood will be inaccessible without the expansion and the new classes/races will be locked as well.

[AlliciaCapulet]

Well, at this time, the CD key is not release yet for ShB, all you have is the early access code if you have pre order.

How this works? The game assume that you have a ShB account until July 6th, starting on June 28th at 5 AM ET. So, you can access every content of ShB, even if you don't have your CD Key yet, until July 6th. The delay is for people who wait for the physical collector edition, because SE post them really late, they are not on the way yet. So if you bought a digital copy, just make sure to enter the key before the 6th of July. Also, you can pre order until the release date so if you buy it on June 30th, you'll get the early access code.

Also, be warned that if you live in Canada, you might end up receiving the physical after July 6th, happened here for HW and SB. If you have someone kind on the SE support center (call number, not online ticket) you might have a key if it happens to you.

[Aylis]

just make sure to enter the key before the 6th of July.

Very important bit here. If this expansion is like the others then if you log out in the new content areas. You will not be able to relog back into the game without either registering the expansions key or contacting support to warp your character out.