I'm not sure if this is what you're asking for?
If you want to change what the reply to address is for all new e-mail, you can change the "Internet mail address" field on the Basics tab of the Location Doc. By creating multiple Location Docs you can quickly switch between reply to addresses. However, an Administrator can defeat this by forcing only the "Internet address" in the Person Doc of the Public Directory be used for the reply to addresses. Then you would need to change your Person Doc information.