Backwards compatible from a parsing-this-message-won't-blow-up standpoint but not necessarily from a semantic meaning-of-each-field standpoint if your devs are

Backwards compatible from a parsing-this-message-won't-blow-up standpoint but not necessarily from a semantic meaning-of-each-field standpoint if your devs are ... clever ... enough. ;) That discipline to commit to an API and meaning for each field and not keep tinkering with it in messy ways is an under-valued and under-evangelized aspect.