If you have an active monthly subscription and upgrade to yearly, then you will only pay the difference. For example, if you purchase a $39/month plan and then upgrade to a $300/year plan on the same day, you will see charges of $39 and $261 ($300 - $39). The difference will change with each passing day, as DataCamp's upgrades factor time passed.
If you have canceled your subscription before upgrading, then you will be charged the full amount for an upgrade. If you still have time left in your canceled subscription, contact DataCamp Support Team, who can reactivate your monthly subscription before you upgrade.