The UUID is for the filesystem. They are references used in place of device nodes. That allows you to, for example, split /dev/sda3 into two partitions, without having to change the old /dev/sda4 to /dev/sda5 in grub's menu.lst or /etc/fstab. If the partition is mounted using the UUID number, the correct partition will be mounted.
If you reformat, consider it a new partition. It is a new filesystem, and references to the old one, either device nodes or UUID numbers will be invalid.
P.S. I think I read your post wrong. You must mean that you are deleting XP and installing Fedora!