Can't add dependant

So I tried to add my girlfriend as a dependant on my medical aid today but it got declined. She is already a member of Discovery health but she is currently on a lower cost plan than what I am on. Because of this, her moving onto my policy is seen as an upgrade which is only allowed during January each year. My colleagues have done the same but because their partners weren't on any medical scheme or on a different one they didn't have any issues. So at the end of the day Discovery is acting against their own customer and apparently even moving or quiting the scheme won't solve the issue because they will pick up that she was a member on her I.D. number. I refuse to accept this.