Hey @Simon,
Thanks for reaching out!
You can perform an upgrade manually any time you’d like. The cost of an upgrade is indeed the cost of the license you are upgrading to, minus the amount you’ve already spent on the license you are upgrading from.
To do this, head to http://theeventscalendar.com/license-keys
Navigate to your Business License.
And then just look for and use the “Upgrade:” links in the top-right corner of the license key box, beneath the “Renew” button.
I already have an Agency license in my account, so for me the upgrade link is only to the “Unlimited” license; but if you have a Business License, then you will see links for Agency and Unlimited. Use the Agency link and you’re good to go!
Here’s a screenshot of where to find those links, for reference:
I hope this helps—happy upgrading!
— George