Another option might be to change the file path entirely in 2.1.x (e.g. to

/etc/ifupdown-ng/interfaces

) so that the update resets the configuration back to default unless the user intervenes by creating this new file in advance. The big download page notice can say "your network settings will revert to default unless you follow __these instructions__" with a link to a page that tells the user to copy the file and make the

s/eth0/br0/g

+

bridge-ports node1 node2 node3 node4 ge0 ge1

edits by hand.