I've seen a specialty provider renumber about 15 class C's _twice_, the first because he changed providers and they urged him to CIDRize. He then had to change again when the first provider didn't live up to the contractual obligations. Because of a lack of planning on his part, he lost 20% of his customer space during/after these renumberings--customers don't want to put up with problems such as a) applications which compute the IP address into an equation with the license number, b) problems with _all_ parameters not being changed in terminal servers and hosts, c) DNS problems with off-site backup servers, and a few others that just generally made his customers not happy.
(Keep in mind that "a)" above generally requires you to purchase new licenses from the software manufacturer.)
The problem, you ask? I don't see very many resources available which help ISP's to renumber more easily--time line planners, reminder lists with commonly-missed parameters, etc.
We could use your help in the PIER wg. The areas you have identified are areas that need a lot of work. (Note that I have renumber large chunks of space three times, 127 class C nets, a class B and part of a class A) If you could help identify those vendors who make use of a), then we could take soem time to wrok with them on other methods. b) is being worked on by Dave O'Leary and c) is part of the Dynamic DNS work. -- --bill