All our shared and storage seedboxes can be upgraded to a higher package anytime from our manager. You will only need to pay the difference beetwen your current account and the account which you would like to upgrade to. The upgrade price is calculated from a credit of the unused portion of the current plan and billing of the new plan for the same period.
As for the upgrade from SH8 package to SH8 XL, we will increase only HDD size and the account will remain the same. In all other cases upgrade requires a new account installation, probably on another server, but of course we will transfer all your files to the new account. In such a situation you will get an email with the information when we started and finished your upgrade.
The downgrade option is not available with shared and storage seedboxes at all, to do this you need to order a new account and cancel the old one.
Dedicated seedboxes cannot be upgraded/downgraded at all. To do this you need to order a new account and cancel the old one.