As others have said, it's pretty much inconceivable that we will become unable to return to the Source at will - even for just a short space of time. The only question is how much time they'll bother to spend explaining how it's possible. Personally, I consider it very likely, once we get to the First, there will be a cutscene that explains that the Crystal Tower has been established as a conduit to allow easy transport between the Source and the First - all before we're able to take even one step onto the First Shard while under our own control.

Quote Originally Posted by Neophyte View Post
Seems like some people already forgot how it was done after our character got 'captured' by the Amal'ja...

a.k.a. 'you're free to teleport whenever you want out of your prison; also you can queue for Ifrit now!'
Amal'Ja Priest: YOU HAVE FALLEN INTO OUR TRAP! OUR GREAT LORD IFRIT WILL SUP UPON YOUR SOULS!!!
WoL: Mind if step out for a bit? My retainers just got back in, and I want to send them out again.
Amal'Ja Priest: VERY WELL, IT CANNOT BE HELPED. BUT DO NOT DAWDLE, FOR OUR LORD'S WRATH IS VAST, AND HE IS NOT TO BE TRIFLED WITH!!!
WoL: Fine, fine. I may run a dungeon, as well; we're getting close to the daily reset.
Amal'Ja Priest: WE BELIEVE IN THE HONOR SYSTEM. DO NOT SPOIL IT FOR FUTURE PRISONERS! THIS IS WHY WE CANNOT HAVE NICE THINGS!!!

And again, later with Ravana!