AOH did not replace the Novice class but since each club can choose if they want to offer these classes many are choosing to offer AOH and not offer Novice.
Here are the descriptions of these two classes.
SECTION 6. The Novice Class shall be for dogs six
months of age and over, which have not, prior to the
date of closing of entries, won three first prizes in the
Novice Class, a first prize in Bred-by-Exhibitor,
American-bred, or Open Classes, nor one or more
points toward their championships.
SECTION 7. The Amateur-Owner-Handler class
shall be for dogs that are at least six months of age that
are not champions. Dogs must be handled in the class
by the registered owner of the dog. This class is
restricted to exhibitors who have not, at any point in
time, been a professional dog handler, AKC approved
conformation judge, or employed as an assistant to a
professional handler. Additionally, members of the
immediate family or household of a current professional
handler are ineligible for this class (as defined in
Chapter 11, Section 13)