I wasn't able to reproduce this. Here are the steps I used:
1. start stable/mitaka OVS agent
2. boot a VM to flat1 (which is a network of type flat with physical_network = 'flat1')
3. stop the agent
4. start the stable/newton OVS agent
5. boot another VM to flat2 (which is a network of type flat with physical_network = 'flat2')
They both end up with different tags. I can reboot the agent as either stable/newton or stable/mitaka and it starts up fine with both VMs.
I wasn't able to reproduce this. Here are the steps I used:
1. start stable/mitaka OVS agent
2. boot a VM to flat1 (which is a network of type flat with physical_network = 'flat1')
3. stop the agent
4. start the stable/newton OVS agent
5. boot another VM to flat2 (which is a network of type flat with physical_network = 'flat2')
They both end up with different tags. I can reboot the agent as either stable/newton or stable/mitaka and it starts up fine with both VMs.
What was the order of events for you?