I am pretty sure that that won't do any good. If you try downgrading a setup with multiple l3 nodes you'll end up with multiple host having the neutron-server assigned (which is something we don't support especially when downgrading to an the old schema revision).

I think the above is the best we can to for the downgrade scenario. Though it only works correctly if you downgrade a setup where neutron-server and neutron-l3 are running on the same host. But do we really need more? I guess downgrading is mostly useful when rolling back a previously installed, probably faulty upgrade.