I don't think there will be a way to fully migrate it like that, but you could try to find someone willing to continue running a Synapse server and ask them to set up a bridge to your XMPP channels (for example with Parsee or the Aria-net Bifrost). That way when you shut down your Matrix server, the existing distributed rooms will continue to exist at least on that other Matrix server and the bridge will ensure that people can communicate with you indirectly.