From what I can tell in this current beta there's simply no build string for the older APs. So, upgrading a network should just not do anything to those older APs. Perhaps in future beta versions it will assign the most current supported version(s) as most firmware releases do as you mentioned.
Ryan If you found this post helpful, please give it
Kudos. If my answer solves your problem please click
Accept as Solution so others can benefit from it.