– If you're asking your users to change the format of something (e.g. from A Record to CNAME addressing), make certain that you know if the continued presence of the old method will prevent the new method from working.
– If it will, be very sure to tell your users to delete the old method as well as telling them to add the new one.
– If you send out alerts requiring your users to take a technical action, users will reply to those emails. Answer those replies. Route them to new support tickets if need be.
– If you change support URLs (and systems), make sure that you disable not only the ability to open new calls but also the ability to reopen calls in the old system. (This has now been done, I see).
– Also make sure that any replies to messages in the old system are forwarded to the new system or are being watched by someone.
– Also, black text on the old system's home page with a link to the new system is probably insufficient. Large red letters are more effective. Automatic redirects are better still. (And that last has indeed now been done as well).
– Vendors make mistakes sometimes or provide incomplete information. Doublecheck the details.
– Even if you don't quite know what they are or how they work, now you know that an A Record trumps CNAME addressing.
– All kinds of crappy stuff can happen to not only the path to your data, but to the data itself. Do backups. Now. Today. Really. No shit. Now.
Update July 13, 2009: other lessons FINALLY learned.