I think (speculate, guess, make wild assumptions) that they want us converted before the beta so that we can test the stuff that has high level requirements during beta with our real characters but not make real progress. Otherwise, we only beta test as many levels as we can gain during the beta, yet SE knows that they need to support endgame on day one. Sorta leaves a vast gap in the testing if they don't let us in with our 50s for at least part of the beta. It'd be a hassle to only convert those legacy players chosen to play beta, so they probably want to convert all of us so they don't need to select beta testers before starting the conversion.