As far as I can tell, I am not aware of any migration path that preserves network connectivity for your VM's; if such path does exists, this may only work in specific settings (i.e. with specific hypervisors, storage, etc).

A more viable approach is snapshotting your VM's into Glance prior to the migration from nova-network to Quantum and redeploying them after the upgrade. This causes some downtime, but this can be minimized if you run the two environments (nova-network based and quantum-based) along-side each other.