Good evening Elinor and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
I probably need to upgrade to events calendar pro. Is it possible to do that, or do I have to pay for a new license completely?
The Events Calendar Pro being a completely separate product from Event Aggregator, yes, you will simply need to purchase a license for it and you should be good to go.
On a related note, we also offer bundles to our customers: https://theeventscalendar.com/bundles/
Let me know if that helps.
Have a great day!
Geoff B.