Basically, we have a list on the wall of features we want to finish to consider AG3 "complete". Once we are "feature complete" as we call it, we go into open beta, open the doors for everyone to come check it out, and then find places to make the site more efficient, perform better, etc., fix bugs, etc.
Once public/open beta has run its course, (and there's no time line, it takes as long as it takes), then we will replace the existing web site with AG3, and AG2 goes away.