Yes, the full cost of the upgraded subscription is due at the time of upgrade, but you will also receive a refund for the time remaining in your previous subscription.
If you upgrade your plan before the end of your current subscription, you will receive a refund for the time remaining in that subscription. If you upgrade within 14 days of first signing up AND within your first year of membership, you'll automatically receive a full refund for your previous plan. After that point, the upgraded refund will be prorated based on the time remaining in your current subscription.
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