Let's not forget to mention the 10,000 dialup customers who have to
change their DNS numbers. We have sent snail-mail and e-mail over and
over again, yet only about 1/3 of that 10k have actually made the change
away from the old numbers. How many of those customers do you think
we'll lose when we officially turn off the old ip's?
Keep a /32 route internally for your old DNS server numbers for a couple
of years. Your customers will still see it, though noone else will. Of
course your entire customer base will possibly lose connectivity
to (1/2^32) of the internet - but they'd probably prefer that to losing
their nameservers. If you want to be really cunning set the nameservers
on the old box to resolve www.yournetworkname.net at a different server,
which has a big notice to change their nameservers, plus a downloadable
executable to go and do it for them on most operating systems. Presumably
you are handing out dynamic DNS by radius in any case, which overrides
the built in settings which copes with 95% of users.