> zipcodes don't disappear that I know of, but the names of the places
> that they describe _do_ change.  (The area where I live recently had
> its postal name changed, and apparently keeping the old name valid was
> _not_ the default and had to be fought for.)

The other change also happens (same name, different zipcode)

The US postal service does prefix-based hierarchical routing (first
three digits selects a "hub" facility; the next two select a post
office within that hub's region; the rest are sorted out at the local
office).

My town got renumbered in the past couple years (02174 to 02474 and
02476), as did a bunch of other towns northwest of boston, when the
postal service did some "traffic engineering" of a sort..

				- Bill