What happens when you change your subscription depends on the subscription you are changing from and to. Since you are free to change your plan during a billing cycle any variations in cost will be automatically calculated to match the time you’ve spent on each plan. This will also be specified on an invoice which you will receive immediately after up- or downgrading.
Because you pay at the start of a billing cycle this can result in a difference between what you paid and what the costs should be after an up- or downgrade. When you upgrade to a more expensive plan you will get a new invoice and your card will immediately be charged to compensate for the cost difference. On the other hand, if you downgrade to a cheaper plan you would’ve paid to much and that difference will be compensated on your next invoice. The date of which can be seen right above the invoices overview on the “Account Settings” page.
More detailed information about downgrading your account can be found in the Terms & Conditions chapter: “Downgrades”.