If you have a valid paid plan, you can purchase a new plan at any time, as long as the remaining period of your current paid plan is less than 183 days. Upon the successful purchase of your new plan, your storage space will be updated, and the remaining period of your previous plan will be added to your new plan.
For instance, if your current plan is 500GB/1Y with an expiration date of 5/10/2023, and you choose to renew with a 1TB/1Y plan on 1/1/2023, your storage space will instantly upgrade to 1TB. The expiration date for your new plan will be calculated by adding the remaining period of your previous plan, making it 5/11/2024.