WoD beta was going strong when I got it in July of 2014 and Talador/Spires/Nagrand weren't open. I'm not even sure you could level to 100 yet.This. This is complete guesswork/speculation on my part butit seems historically, the transition seems to happen once the most of the main stuff is in. It's getting closer and closer with each alpha build pushed. If i remember right with some past alphas, it seemed to go from alpha to beta a build or two after the max level for the xpac was obtainable/all launch zones were open. Currently it's at 108 at still a few zones not yet open. part, but just something that I've noticed in the past with other alphas.
Got the Alpha email today too, so much be another wave of invites.