There won't be any need to reroll; all characters will be changed over to the new system when HD gets released. Most likely, any existing characters will automatically get a certain amount of trait points to spend in the new tree (depending on level and completed class deeds, as I understand it), and you'll be able to set your character's traits and specialization immediately. If some traits/skills are gated behind class deeds that you haven't completed yet, you'll have to complete the deeds before accessing those.
Please note I'm not part of the HD beta; I'm speculating based on what's been described publicly and my expectation that it will work a lot like the existing mounted combat trait trees: when U11 revamped the mounted combat yellow trait tree for all classes, everyone's mounted trait points were reset, but the total amount of points available to a character didn't change. It didn't matter if your account was VIP, or whether you had purchased Wildermore QP separately -- the mounted combat changes went into effect immediately, for everyone.
The class changes in HD will be similar: when HD is released, everyone will use the new class trait trees. The class trait slot system we use now simply won't exist anymore.
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