What usually occurs, though, is someone begins to use autonumber as a
sequential numbering device, and then decides that all numbers must be
consecutive -- no gaps allowed. And then the person decides he/she wants to
change a number for an entity, and wants to know how to reassign all the
numbers (which should not be done in any situation involving the primary
key, regardless of whether you use an autonumber or not as the identifier).
It's possible to use the autonumber as the "hidden" primary key and then to
generate (even automatically) a visible ClientID that can be enforced to be
consecutive, with no gaps, and even could be "changed" if desired -- all
without affecting the very important primary key value.