The change in syntax from:
protocols { bgp <ASN> { ...
to:
protocols { bgp { local-as <ASN>
did not happen automatically on my configuration and the failure was entirely silent.
As a result, after reboot, my router (which is remote and depended on BGP to be reachable)
ended up loading none of the BGP configuration stanzas and I have had to manually reconstruct
them from the pre-migration file.