It might be wise to keep the old addrs as host routes on interface
aliases on the same machine for simplicity sake. (Joe said that kinda).
Both unix and cisco support this. You will likely not miss a beat
if you're able to do this and see who's using
the old addrs(hard coded) after the TTL expires - methinks.
If you really care, you could chase down your hard coded users
or just shut down and force them to call. The number would dictate which
one I suppose.