Upgrading a Digi Fibre plan is allowed. The contract will be carried forward to the new plan for upgrade within the Digi Fibre plan. Upgrading from a legacy plan to the Digi Fibre plan will refresh your contract to 24 months. Upon upgrading, the bill will be prorated based on your usage of your previous plan and the new plan. You may proceed to walk into the nearest Digi Store to upgrade your Digi Fibre plan.
No downgrade of Digi Fibre plan is allowed at the moment. In order to proceed with the downgrading of your Digi Fibre plan, a subscriber is required to terminate the current Digi Fibre subscription, pay the penalty charges and sign-up for a new Digi Fibre line.
Refer here for more information about the contract, early termination and penalty.