You can change your subscription anytime.
Upgrade and downgrade
If you will upgrade or downgrade, your card will be charged immediately and from that moment, the new month will run. Your operations and data limits from the previously active plan will be converted to Extras and placed to your account.
Extras are transferred from month to month and never expire. Extras are used only in case you will hit your prepaid monthly limits.
If you don't like this conversion of your prepaid limits to Extras, you can wait with this change until your current prepaid period ends and resubscribe after.
Related topic: How to change a card used for the subscription
If you will cancel, you won't be charged again and your current plan will remain active until the end of prepaid period.