You can upgrade and downgrade anytime:
If you want to upgrade, the time remaining in your subscription will be discounted from your purchase. The purchase price of the new plan will also be pro-rated to the time remaining in your subscription, so the date of the renewal stays the same. For example, if you upgrade after 6 months, you would be charged 6 months of the new plan minus 6 months of your current plan.
If you want to downgrade, we apply the same principle but we do not refund the remaining credit: it will be discounted from your next purchases instead.