Comment 1 for bug 1533615

If we check the other object update operation implementation in Neutron, we can find almost all the object's list update will use the new replace the old.
eg,
1. create object['attr'] = [a,b], then object['attr'] = [a,b]
2. update object['attr'] = [c], then object['attr'] = [c]
3. update object['attr'] = [a,b,c], then object['attr'] = [a,b,c]

There are many object implement as this behavior above.
eg,
port object's security_groups list,
subnet object's allocation_pools, dns_nameservers, and host_routes list.
router object's external_fixed_ips, routers list
...

So, it's acceptable to replaces the new flow classifier with the existing one unless specified both existing and the new flow classifier in the port chain update.

We can discuss more about this.