MNT ROBBING THIS PENSIONER

I am a pensioner who signed a contract with Mtn about 4 years ago...I paid R280 odd a month for keeping my own phone and sim and they gave me 5gigs of data plus 100 free minutes a month...not sure on sms never use that...since then Mtn occasionally sent me sms about changes to my contract ..these I never acted on..mainly becos I know how frustrating it is to deal with call centre's..so after constantly adjusting my contract and premium I am currently receiving 1.5 gigs data and 100 free minutes that costs me R419 pm...on visiting mtn clearwaters this week they show me that Mtn is only charging R209pm for a contract where you have your phone and sim and they give you 1.5 GB and 100 free minutes..so why are they robbing like this...is it legal for MTN to change my contract without my consent??I called MTN yesterday to try and understand this and after hearing my storey customer care decided I needed to go to retentions to change my contract...after hearing my storey retentions decided I needed to go to business retentions...after a few rings here I cut the call...that call was 15 minutes...frustrating...as a pensioner I just want to be treated fairy by MTN...IS THAT TOO MUCH TO ASK FOR??