You can upgrade at any time. Your websites, mailboxes and databases stay as they are: only the limits and resources change, with no downtime.
How to do it
Open a ticket from Support stating the plan you want to move to. We will tell you the amount (you only pay the difference for the time left in the period you have already paid) and apply the change as soon as it is paid.
Downgrading
This is also possible, as long as what you have fits in the new plan (space, number of websites, mailboxes and databases). Write to us and we will apply it from your next renewal.
Just need a little more?
If you only need extra space occasionally, first check how to free up space: it is often old emails or backups stored inside the website.