Lets say the document is named 'Fred'
Well, users can open the document and use 'save as' to call the file: 'Fred1' - then from windows explorer rename 'Fred' to 'FredOld' and Fred1 to Fred. voila. The protection is by-passed.
So, the protection will work for 'some', perhaps even 'most', but not too well against any computer savvy user...