they did not cheat or coerce you when you signed up for their plan few months ago. it was based on willing buyer willing seller basis.

circumstance has changed since then with other telco throwing prices , Digi has to respond accordingly.

I don't see any fraud or malpractice , it's dynamic business environment.

it is not limited to telco. even supermarket gives fantastic offers from time to time. can we cry foul at that scenario too?

there are remedies
1. terminate the current contract and pay a penalty fee
2. write in to Digi to ask them to upgrade your line as a courtesy to you, not an obligation.
3. remains in prepaid line forever in future / or direct purchase your phones with cash instead of some easy payment or telco subsidies.

welcome to the real world where contracts have to be honored