If a user really wants to be able to downgrade to 2.0, they could add back the

eth0

config block alongside the

br0

block. I think that would mean 2.1 ignores the

eth0

block and 2.0 ignores the

br0

block so both configs can coexist. But if there's no

eth0

block then 2.0 would just not bring the network up. The user would probably want to go into safemode and fix their config.