What happens to 2-way airport express travel pass if we're staying in Hong Kong for more than 3 days?

we want to buy the 2-way airport express travel pass bcoz of the unlimited mtr rides, however we will be staying for more than 3 days, can we have it reloaded for another 3 days of unlimited mtr rides? or the return airport express ride will be forfeited? thank you for whoever will reply...