If you made it a new partition with the label (mount-point)/usr you probably did just as the above post suggests. If you know the partition number of the new partition, you can umount it as /dev/hdXx. This would be "safer" than using a volume name. Then e2label the partition to something else--say /usr2-- if you are planning to extend or copy /usr to it.
If it refuses to umount because it is busy, boot up to the partition magic and relabel the volume. Then, you will have to edit /etc/fstab (check /etc/mtab as well).
After changing the label on the volume you may want to reformat it to be sure everything is OK. That is, unless you already put some new files in there.
There should be no damage to the existing /usr unless you did something you didn't say.