Yes if you upgrade within the promotion period, you’ll get to enjoy the promotion attached to the new upgraded plan, be it bill rebates and/or discounted devices.
However, you will forfeit the discount you initially enjoyed and your 24 month contract will be reset.
eg: James upgrades from 100Mbps to 300Mbps plan . James got a discount of RM10 x24 month for this upgrade. James then decided to upgrade to 500Mbps, he will now STOP getting the RM10 x24 discount. He will now get a new discount for 500Mbps plan.
If you upgrade your plan after the promotion period, any discount you may be enjoying will continue with no contract reset.