Existing clients pay more money

On their website, they advertise 100Mbps/50Mbps for R519. I am currently paying R757 for 50Mbps/50Mbps. When I wanted to be transferred to a cheaper faster subscription, I was told that "Unfortunately, the T&Cs only allow you to change from 50/50Mbps to 200/200Mbps, which is R1 097.00pm, but the Black Friday promo price will be R827.00pm"