It seems that they aren't expecting anyone to switch over at this point. Only users of the VivoFit have access to the modern Garmin Connect, and from what I understand activities from non-VivoFit devices don't appear on the modern Garmin Connect. If that is indeed the case, then they are expecting you to switch back and forth. When they make it available to users of non-VivoFit devices, I think they'll have more of the classic functionality available. If it isn't there initially, I think you can assume that they'll be trying to provide it.
Travis