When you change your plan, the rate is charged pro-rata. In short, this means you can change your plan at any time without any penalty at all. To do so, simply navigate to the Upgrade page in Settings (accessible by clicking on your avatar within the top bar) and choose the desired new plan.
To further clarify, for example, if you’re on a plan costing $29 per month and, halfway through the month, you’d like to upgrade a plan costing $69 monthly, you’ll be credited half of $29, and charged half of $69 for the remainder of the month. This would equate to a charge of (69/2) – (29/2) = $20.
Evidently, the same process occurs in reverse when you downgrade from one plan to another.
For example, if you had one week left on the $69 plan and downgraded to the $29 plan, following this, you’d be credited for one week of the $69 plan. That credit would then be applied to one week of the $29 plan, meaning that there would then be some credit left over which comes off your next bill 🙂