Managing updates and versions

I have a question about managing happ updates and versions in holochain ecosystem.
If I switch to a new version of an HApplication, how can I be able to communicate with other agents who still have a lower version? Also, what happens to the data history (old version of DHT and Source chain)?

maybe you’ll get pinged on this anyways, but I made a response to a super similar question here: Updating the source code is going to create a new chain? - #2 by Connoropolous