I am fairly certainly you must reload the configuration with the new hardware settings. A restart may also be required. Of course, it is always advisable to halt the operation controlled by the PLC before making any changes as there is always the chance of an error(s) that could inadvertently force the PLC into STOP Mode.
Do you have the current password? If not, there are several options but they all involve destruction of the current running program on the memory card, reformatting (with the Field PG) or replacing the memory card altogether. Based on my past experience with near disasters, any password work-around should be tested on a separate PLC with a Separate Memory Card so as to not adversely affect your current setup. Also do not try to reformat Siemens Memory Cards without a Siemens Field PG.