The no-prepend and local-as options have been made available to support the 'neighbor a.b.c.d local-as' feature.
Put into my own words the 'local-as' feature allows internal as numbers to be changed, but still allowing partner BGP routers to peer with an old AS number. Hence this enables BGP AS to be changed without affecting partner BGP configuration.
One downstream affect of this is that the by default partnering AS systems still see both old and new as numbers in the path.
With the no-prepend option the 'local-as' number is NOT prepended to downstream BGP neighbors (except for the partner who is peering with the 'local-as' number).
With the 'replace-as' option the real bgp as number is not sent to the BGP partner that peers with the 'local-as'.
I found an excellent article on this at
http://wiki.nil.com/Network_migration_or_merger_with_BGP_Local-AS_feature
