How do I change my master password when the current password is to press enter (with nothing in the password blank)?

Whenever I try and change my master password I can not manage to. I got a new hard drive and the person at the apple store told me that the password was to just press enter (with nothing in password blank), and that I would be able to change it. Everytime I try and change the master password it says, "incorrect password" under the current master password blank. It has been awhile since then and I have moved so I can not go back there and get them to figure it out. So the question is, Is there anyway to change it without wiping my entire hard drive and starting over?