1. What is the benefit/goal of upgrading to 6.3? (so that you can weigh it against the time/expense/effort of the upgrade)
2. Why isn't your data backed up already??
3. Have you seen this? (1st Google hit for "upgrade CentOS"): http://wiki.centos.org/HowTos/MigrationGuide
4. There may be additional migration documents available at RedHat.com (CentOS is based on Red Hat) although as far as I know Red Hat does not officially support upgrades across major releases.