Absolutely! If your in need of some extra storage just click the Extend Account on the top of your own SQZFILE page. The account will be upgraded instantly from the moment you make the initial payment.
If however you wish to downgrade, simply follow the same instructions as above and you will be downgraded to the selected account from when your next payment was due. This means you will remain with the previous amount of storage space until the payment date.
Please note, once you have begun your new downgraded account you will be required to remove files to stay within the usage limit of the new account if necessary.