Yes, you can upgrade your license. If you are using a $29 license and want to upgrade to a $99 license, You will pay $79 only to upgrade your license.
Go to your-account. First, You need to login, click on the ‘Purchase History Tab'. Check the last column ‘License Keys'. If you have more than one rows in the table, Choose the product which you want to upgrade. Click on ‘View Licences' of that product. The next page will appear with a table. Check the last column of that table “Upgrades“. Click on it and start the checkout process.
that's it. Easy ??