I-Novae have created their own user management system and this appears to be something that simply hasn’t been implemented yet.
Discourse by default allows users to change their email address but, because of the integration with I-Novae’s custom user system, this is disabled here.
Depending on how busy @INovaeKeith is he might change it for you but I can’t promise anything.
The Add Email Address page does not allow you to change your primary email address - it allows you to add secondary addresses. I will be adding a mechanism to change it at some point but at the moment I have to do it myself.