You can always upgrade from one license to another and you only pay the difference. There are basically two ways of doing it.
Go to the product page of the plugin, scroll down to the pricing section and click on “Buy Now” and then on “Renew or upgrading license?”
Once you added your current license key, you will see the remaining payment due directly within the checkout.
You can also upgrade directly from within the account area here. Log in and go to “Licenses”, select the license you want to upgrade, and click on it.
You will find a little select menu in the right corner that says “Upgrade to”, select the desired license and complete the checkout to upgrade.