Hi- I think a lot of times it is just breaking the habit. My 3.5 yr old went through this about a year ago. I tried everything, even bringing him into our bed to get him to fall back asleep. He was, but he started to get comfortable in our bed. So finally, when he woke early, I told him that mommy isn't coming to get him and he can play in his crib or go back to sleep but it is not time to get up and I am not coming to get him until it is time. We went through about 2 mornings of him crying and then every morning he was quiet. Eventually he stopped waking up so early.
Like I said, it is breaking habits. You have to have nerves of steal to listen to them crying but it is just "re-training"
Good luck