In beta, you may switch to any class at any time.
The caveat for Legacy players is skill sets still are locked till you complete your starting class quest line to the level 10 quest.
It's behoove Legacy players to complete the quest lines in full, though, as each class has two abilities obtained only as rewards at 15 and 30.
There are other things locked behind quest lines as well like Inns, leves, retainers and Grabd Company membership.
That's fine. In my group we are split down the middle: 6 legacy, 6 new or starting new. Figure we're going to post 2 legacy players in each city so they can switch to whatever class is needed when the new players get to the level 15 dungeons.