$25 Go Big discount credit vs $25 Mobility credit

I was an original subscriber to DirectvNow when it launched.  I received the Go Big discount on my plan and was promised the lowest rate possible for life.  At the time this discount was not tied to my cellphone service.  This was November 2016.  However in 2018 my discount was changed to a mobility credit and tied to my cellphone account.  Now when I try to change my cellphone plan with Att, I am told that if I change my plan I will lose the discount (which never should have been tied to my cellphone plan.).  Being a customer with ATT was not a requirement when I signed up in 2016. 

Who can I speak to about this?  Why was my credit and account tied to my cellphone plan?