In order to make the instance conform to the class Cto, local slots specified by the class Cto that are not specified by the class Cfrom are added, and local slots not specified by the class Cto that are specified by the class Cfrom are discarded.
The values of local slots specified by both the class Cto and the class Cfrom are retained. If such a local slot was unbound, it remains unbound.
The values of slots specified as shared in the class Cfrom and as local in the class Cto are retained.
This first step of the update does not affect the values of any shared slots.