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When you change your password, your private keys are erased and replaced with new ones. Occasionally it's useful to have your old keys for some reason. This happened on Bitshares recently when Peerplays launched; Bitshares private keys could then be used to claim a balance on Peerplays. Anybody who had replaced their old Bitshares keys would lose the opportunity to claim free Peerplays.

Good to know. Thanks!