It easy to say unless one is impacted..how many read terms and conditions..
Not many read the terms and conditions, but in your case it seems that you made an assumption on what should be included based on other plans, not that specific plan.
it is not fair to sell plan that doesn’t exist
Did Optus/Retail store make a misleading claim about the specific plan you purchased? If so then you can certainly argue for cancellation based on that.
the contract says plan can be changed
Then all you need to do is go back to the store and show them where in the contract it says that. But I'm guessing that your actual plan change options are limited in the contract and the plan you want to change to is excluded based on those conditions.