I think they had a rebuild option in the 3.X Living Greyhawk guidelines packet for when new sourcebooks came out, as long the new rules in the sourcebook were approved in the latest packet update -- if that's the case, it wouldn't be unheard of to allow the rebuild when the new core Player's Handbook finally released. One of the points of the "four core classes" is to allow a fair amount of flexibility within each for multiple archetypes at the "Basic" level -- the classes outside of the four are more specialized, or "Advanced," versions of those various archetypes, which a character could theoretically rebuild into after the Player's Handbook releases.
Living Greyhawk allowed you to swap out a single feat or spell if it got errata'd heavily. That option was only available late in the campaign and only major changes triggered it. If there were only small changes and you were stuck with what you had. Later, the campaign also allowed the PHB2 retraining rules, but that meant you could swap one feat every time you went up a level and it cost money. All characters still had to be created at level 1 and you could only play adventures written for whatever physical location you were in.
Living Forgoten Realms was a lot more open and let you rebuild your character entirely except for name, race, and class every level you went up. You could also create characters at 1st, 5th, 8th, or 11th level. The rebuild option and creating characters at higher level weren't available early in the campaign, however. A large number of the administators and player base didn't really like the idea that your character could completely morph into something else every level. However, the higher ups said they needed the campaign to be as open as possible to allow for a lot more casual players so they changed the rules partway through the campaign to add these rules.
I've heard rumors that the extreme openess of allowing complete rebuilds turned away a bunch of people and caused as many problems as it solved so that there was thinking within the leadership that the next campaign might need to be a compromise between the two systems.
How this ends up panning out regarding rebuilding and how easy it will be in the new campaign is anyone's guess except the admins of the new campaign.
I just think a lot of people will be frustrated if the Starter Set includes only 4 classes or not all the races and we're forced to play a race or class we don't want at Gen Con. Especially if they won't let us apply that xp and gold to a different character we create later.
Though, as I said in my previous post, I think it is well within the realm of possibility that they allow a one time rebuild or the ability to apply xp and gold from Gen Con to any character at all. Though, I think they wouldn't go into Gen Con with that as the only option. If we don't get a PHB at Gen Con, I think it's much more likely that the PHB was planned for Gen Con months ago and sometime in the last month someone made the call to delay the PHB because they couldn't make the deadline.