Much thanks to all who responded. I seemed to get a pretty consistent response from everybody who replied: Your Milage May Vary. Some people had had no problem with getting renumbering time, some people had had tons of problems. Some people said MCI had been very reasonable, some said that CW changed their policies. At any rate, once I knew I wasn't being that unrealistic, I emailed the IP allocation people at MCI, and got this response (which they asked me to post back to this list):

Scott,

This is the correct email address for such requests. The only problem being the length of time you are requesting. Our normal policy is to allow 15 days which can be extended to 30 days. Of course this all depends on the number of addresses and types of applications that require renumbering. Please send all the information regarding this request to include circuit ID's of both customers to ipadmin@cw.net. Also I would like to know who you talk to at Cable & Wireless that gave you the impresssion that we were unwilling to work with our customers. Please do me a favor and spread the word on all the mailing lists that are bashing us right now that we will negotiate a reasonable time for renumbering for any customers that leave Cable & Wireless (with emphasis on reasonable). Thank you.

(I have omitted the name and telephone number of the technician that sent this response). So it sounds like our customer just talked to a butthead salesperson who didn't want them to know what they could do. Thanks again, --------Scott.