If you have a valid paid plan in your ASUS Cloud ID, you can purchase a new plan on Plans page when the remaining period of your paid plan is less than 183 days.
After your purchase is successful, the storage space in your ASUS Cloud ID will be replaced with the new plan. The remaining period of your previous plan will be added to your new plan.
For example, your current plan is 500GB/1Y, and the expiration date is 11/10/2022.
After you purchase a 1TB/1Y plan on 10/10/2022, your storage space will immediately become 1TB, and the expiration date of your new plan will be 11/11/2023.