You don't have to issue a CHANGE command at all if they've already
subscribed under their new address -- just delete the old one.

There is one special case where this is confusing: when they've never
subscribed under the new address and the old address is equivalenced via
the LISTSERV alias facility.  E.g., if I were subscribed as
[log in to unmask] and want to change that to [log in to unmask], the CHANGE
command would say that I'm already subscribed since we do in fact have
that host address aliased.

/Pete