With any regular shield, typically one makes bludgeoning damage with a shield bash. If you add shield spikes, it changes the damage die and type to piercing. Would it be possible still use the altered shield for a lower damage bludgeoning attack if so desired?
These spikes turn a shield into a martial piercing weapon... (d20pfsrd, emphasis mine.)
The shield becomes a martial piercing weapon instead of a martial bludgeoning weapon. It effectively loses its bludgeoning key-word and changes type.