2.2 kernel to 2.6 on a live public facing webserver

There are quite a few supporting-surrouding packages which need to be remain those against which glibc was compiled, not those of the current kernel, RedHat (and possibly other distros) of that vintage used a symlink intousr-src-linux, which was not correct. After that, most of the issues are those standard to moving to a new kernel.

Yes, that's not necessariliy the best way to do it, because of the cached nature of DNS - you can have some clients hitting the old server and some hitting the new server at the same time, and if there is any dynamic content, that can lead to data loss and-or confusion. You could start out with a really short TTL, but then you burden the client resolvers with frequent lookups, and you can still run into some odd DNS cache which doesn't honor your TTL. With 1-hour TTLs, I've seen some client resolvers keep the records for as long as 3 days.

You could do IP-based hosting, which solves the issue, but comes with it's own set of hurdles and work as well, as does trying to uprade the machine without moving any. There's no easy answer.

pointed out, you can find with Google.

If the operation involves any significant sort of revenue from the machines, then try to always err on the side of safety. It's usually easier to explain to your boss why the project took longer than expected than to explain why you borked 2,000 client's websites!

Also, if your systems are still on 2.2, look into whether the rest of the installation has been kept updated. Many (most?) 2.2-based distributions haven't had updates issued in quite some time. If your distro fits that description, then consider a newer OS for security's sake.

steve