I think that Pierre Chang will die or has died preventing the leak at the swan station from becoming too devastating.
"I was just making sure your father does what he's supposed to do." - Daniel to Miles
This goes back to the whatever happened, happened concept. "Variables" can't change anything, but they do a lot of things to help the future along. This has been reccuring in lost: People helping along the future. For example, Daniel making sure chang does what he's supposed to, Eloise encouraging her son to go to the island, Hurley writing Empire Strikes Back, Kate helping the young benjamin linus knowing he will be alive in the future. All of these things already happened and are part of history. Although Daniel said that "the variables" are in their own time and have free will, everything that they do already happened. Eloise knew that she had killed her son in her 1977 past. The swan incident will go on as it was supposed to. Pierre Chang, who is the one ordering everyone around about drilling close to electromagnetic pockets, is the one who stopped the leak from becoming a major catastrophe, while dieing for the cause. It is likely that he was able to do this because of the warning from daniel, which was part of Daniel's destiny to do so. Pierre Chang seems like a good man and a good father. He wouldn't abandon his family like Miles' mom said, although she might not have even known about the circumstances of his death. She said that his grave was at a place where Miles could never go: the Island. It's possible that Chang died in the purge in '92, but unlikely. We've never seen any proof that he was alive after '77. Someone had to save the swan leak from becoming huge. **(it's possible that Miles and his mom didn't leave the island in '77: Mile's powers from radiation, his mom having cancer) The Incident happens, its just not nearly as devastating as it could have been. Charlotte also probably is affected by radiation due to the incident.
It's also possible that Chang had access to Daniel's journal. Jack and Kate couldn't do anything themselves to stop the Incident, but they could have provided chang with the journal. This was all part of all of their destinies. Also, I don't think the hydrogen bomb will come into play. Daniel was trying to evacuate the island and detonate it to stop a chain of events from happening: The button having to be pushed, Desmond failing to push it on September 22 2004, Oceanic 815 crashing, Widmore's boat w/Daniel and Charlotte coming to the Island, etc... All of this was supposed to happen and cannot be changed. Daniel was crazy to think so and want to detonate that bomb. Anything the "variables" do is still part of history.