In principle, there is an original copyright and a copyright of the revised version, but this makes no difference in the duration of a copyright of a natural person which is 70 years from the death of the author. It also has no effect on the duration of the copyright of a corporate work where the legal author is not a natural person, which is 95 years from first publication or 120 years from creation, whichever is shorter.
The main reason to update the notice is that giving a copyright notice is one factor necessary to obtain special remedies such as statutory damages and attorneys fees, rather than merely compensatory economic damages. If you don't update the copyright notice for a new version, then these special remedies won't be available when it is the new material that is copied in violation of the copyright law.
Updating the copyright notice used to be a much bigger deal when the duration of a copyright was 26 years from publication, and renewable only once for an additional 26 years, as it was pre-1978. Under that regime, a claim of copyright was necessary for a copyright to be present and it was only available for published work, unlike current law under which a copyright with minimal protections arises automatically by operation of law upon the creation of the work.