The only changes are that eth0 is renamed to br0 and that there's an extra

bridge-ports

stanza. This should be simple enough to apply the changes directly to a user's interfaces file. But, yes, there will be that one person who has modified the interfaces file a lot more extensively than an automatic migration can handle. However since triggering a reboot before the "postupgrade" script can run (late in the boot process) will cause a rollback, it's possible to have a conservative migration script that performs the change only if it knows it won't break anything, and triggers a rollback (so the user can manually intervene) if not.