Beastlords are their own class, ie predator, rogue, bard, not a class in terms of ranger, assassin, brigands etc.
There is no alternate to BST. Betraying is just going from good to evil or evil to good. A class change is just something that happens because of mechanics. Short answer is Beastlord has no counterpart and will not change classes while finishing Betrayal an no your level 90 brawler/scout/shaman cannot become a level 90 BST on December 6th.
GeminiStar wrote: Yet. I can envision, in a future expansion, their alt class. The Engineer, sort of a tinkerer on steroids. The engineer can create his own golems for attack and defence and would be a ranged weapon using debuffing class. I'm sure they will find some way to distinguish this class from 'chanters and their ilk.