Can I Upgrade to a Higher Plan and Pay the Difference?
Yes, definitely. When you upgrade to a higher plan we will prorate the remaining credit on your current subscription and minus that from your new plan. You will see how much credit is prorated when you bring up the Plans tab.

Note
Your billing period will reset to the day of your latest upgrade.
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Can I Upgrade a Free Campaign That's Already Live?
Yes. You can upgrade your plan at any time, even mid-campaign, and the new features are available on your live campaign straight away.