How does payment work when upgrading and downgrading?

You can upgrade or downgrade your plan at any time. 

How does this affect how we bill you? 

When you upgrade or downgrade your plan you'll automatically receive a refund for the part of the payment term you have not used on the old plan, and a pro-rated charge on the new plan.

What does this mean in practice?

Using round numbers to make it simpler, here is what happens:

If you're on plan A which is $10/month and you switch to plan B which is $15/month halfway through the billing period:

We'll refund $5 (the unused portion of plan A) and we'll charge $7.50 (the pro-rated plan B amount for the remainder of the period).

This will result in us actually only billing the difference ($2.50). After that we'll bill the full plan B amount each month.