Your existing Unlimited Mobile Internet plan will be replaced by your new Unlimited Mobile Internet plan. Any unused quota and minutes will be forfeited, and not rolled over to the new Unlimited Mobile Internet plan.
For example, if you are subscribed to UMI 50 and purchase UMI 30, any unused quota and minutes from your old UMI 50 plan will be forfeited when you purchase UMI 30. You will only be able to use the allocated data and minutes of your new UMI 30 plan.