You can't change it. RIM made it default with all new 5.0 OS and the lock key will just lock the keypad and the mute button will no longer put the phone on standby.
the only way i've found to lock/password the whole phone since the update is by clicking the "lock" icon that's on your homepage of icons....
Do you have a lock icon on the A key on the keyboard? If so all you have to do is hold down the A key to lock the phone. If you do not have the icon on the key still try it anyway.
I say this because the new 8530 Curve II has 5.0.X OS and this how they lock the phone now.