You can change your conference PIN codes as often as you like.
Please make sure that you have no future events scheduled, as the pin code reset will take effect immediately and may cause confusion if you have already published a future event. Also, keep in mind that both codes must be reset, not just one or the other. Once the pin codes are reset, the old pin codes are gone for good.
Please send a message through the support system if you want to have you pin codes changed, or Submit a Request Here.
You also might try setting a conference password on your phone line or webcast template. See this FAQ on setting passwords.