You can change your subscription plan at any time.
The behavior when switching plans is as follows.

For iOS

Upgrade

When upgrading to a higher plan, the upgrade takes effect immediately, and you will be refunded an amount based on the remaining days of your original subscription.

Downgrade

When downgrading to a lower plan, your current subscription remains active until the expiration date, and will be renewed at the lower plan’s price after the renewal date.

Crossgrade

This refers to switching to the same plan with a different validity period. In this case, the new plan will take effect on the next renewal date.

For Android

Upgrade

When upgrading to a higher plan, the upgrade takes effect immediately and you will be charged the full amount of the new plan right away. The remaining value of your original subscription will either carry over within the same entitlement or be prorated when switching to a different entitlement.

Downgrade

When downgrading to a lower plan, your current subscription remains active until the expiration date, and will be renewed at the lower plan’s price after the renewal date.

Crossgrade

This refers to switching to the same plan with a different validity period. In this case, the new plan will take effect on the next renewal date.