Tickets can be revalidated or reissued. AFAIK revalidation is only for date changes/retimed flights whereas reissue of the ticket is mandatory in case of significant changes, i.e. routing, carrier, etc. HOWEVER, from experience many airline agents will simply reissue the ticket even in case of date changes. It seems that to many agent revalidation is not an option.and they will reissue the ticket even if revalidation would have sufficed.

With revalidation the ticket No. doesn't change. With reissue a new ticket is issued (hence new ticket No.)