i would urge you not to use the commands given there, they could easily damage file locations / symlinks etc. and mess up special files.
use something like
find /usr -xdev | cpio -pvmd /mnt
instead of a risky recursive copy.
using the general method you will not lose ANYTHING, you will simply duplicate it. when you mount a file system over an existing directory the data on the directory is simply hidden, and perfectly safe even though you can't get to it. you're using the umount command as you can only mount the device once.
if you are happy that the system is tehn working fine, run "init 1" to drop down to single user mode and the umount the /usr partition and "rm -rf" it to free the lost space up, then just remount /usr. of course you'll need an entry in your /etc/fstab file as well in order to always mount it at boot time.
there's lots of very similar questions around here if you look.