As someone who tried Final Fantasy XI briefly, I would strongly advise learning from the mistakes made there, in regards to "unlocking" new Jobs. I found the initial available Jobs in FFXI to be bland and uninspired, and as much as I wanted to play that Dark Knight and Blue Mage, I simply couldn't force myself to play a character up in order to unlock them, despite having purchased all of the (then available) expansions.
The reason people get excited about Classes and Jobs has nothing to do about "what their buttons do", but rather the sorts of themes they revolve around. A lot of people will be picking Heavensward up because, in all honestly, they probably just love the idea of a Dark Knight. I played a Death Knight in World of Warcraft, and I know a lot of others who share a similar taste, and might find themselves bored with the 1.0 Class selection.
While I think the 1.0 Classes absolutely provide something to the game (and I seriously hope they are refined, to still be useful even after unlocking the current Jobs), I think it would be a huge mistake to force players into playing them from the start. I highly recommend changing your plans, even if it means adding more "Classes" to cover their pre-Job incarnation (ie. "Acolyte" as a precursor to the Dark Knight). Otherwise, you risk alienating the very players to whom the new jobs are supposed to appeal.
Obviously, this close to launch, it's asking quite a lot of the Devs to completely rework their plans. So, I suggest billing it as a "head-start" for current players, accessing the new Jobs from the get go. However, in the next major content-patch, make the new Jobs (or again, lesser "Classes" based on them) available for new players.
Personally? I'd really dig having a new Starter-area entirely, as a small Au'Ra/Doman refugee-camp on the coast of Ishgard (just a small taste of the Expansion, before getting sent to join the other Grand Companies at lvl 15), or even carving out a small section in Mor Dhona to level them. Barring that, of course, you would simply add them to existing City-States (Astrologian would fit right in with Gridania, not sure of the others).
Either way, I think this is something that could really wind up hurting the game, as I feel it did FFXI, if left for too long.