If you must have an identifier that does not have gaps in sequence, autonumber is not ideal. It can be done but is a pain to implement and manage.
As ssanfu links explain, autonumber field is not intended to have meaning to end users, only to db designer when setting relationships. End users should not even be aware they exist, don't even have to see them. Gaps should be irrelevant.
If you re-build autonumber key field, then parent/child relationships for existing records are broken and data is corrupted.