Most of subscribers use personal identification number (PIN) codes which combined with 6-8 numbers to protect their Subscriber Identity Module (SIM) cards from illegal accesses. To enhance security, many previous studies have employed keystroke dynamics to protect the PIN code, and the relative results exhibit that keystroke dynamics can indeed improve the security of a PIN code. However, the traditional keystroke dynamics-based authentication (KDA) system has to collect user's keystroke dynamics firstly and then produce a unique personal biometrics. It's inconvenient for users when changing their PIN codes is required, because the corresponding KDA systems have to be retrained. To solve the above-mentioned drawbacks, this paper proposes a novel technique by which users can change their PIN codes anytime, without any extra retraining. Conducted experiment results show that the proposed approach can effectively improve the KDA system to distinguish legitimate users and impostors even when users change their original passwords.