how can I not be allowed to upgrade the plan when

I have changed jobs in January 2016 and the new employer requires me to be on classic comprehensive but discovery is refusing this, saying the upgrade period has passed. I consider this as extremely unfair to me because I want to upgrade purely because I changed employers and discovery won't give me the option of opening a new cover either. if I had an option, i would move away from discovery completely and get another cover from a different company.