I have a shared server having centos 4. I have to upgrade it to 5 without any downtime. So, what I decided is that to migrate all data and IP to temporary server. Is it possible without any downtime or anyone has any solution over this.
You first have to copy all the contents of the server to an another server which is having another ip. You then have to change the A record for all the domains on the server. You have to wait till the propogation delay is over. In most case it is 24-48 hours depending on the TTL value. After the propogation delay is over you can upgrade your OS and then repeat the steps as before i.e first copying of data, changing dns. You can remove the temporary server after the propogation delay is over.
you can get rid of the DNS mess if you can IPKVM to the boxes. Just enable the correct IP to the temp server after you moved the files from the original server to the other.
One thing i would do first is to set up the temp server on centos 5, transfer the files over and test it, before you actually cut it over to make sure there's no conflict with the packages, etc. etc. Then once you are satisfied with the functionality, just switch the IP - no need to mess with DNS. BTW - instead of getting a temp server, just get a new shared server with centos5 already installed.
You would be better getting a new machine with CentOS 5 and migrating all of your accounts direct to it. Upgrading through a major release version is going to be a painful process unless you just wipe the old server completely. And if you are going to do that, then why go through the process of migrating your accounts to the temp server then going through the same process again to migrate them all back?