Howdy Markus and thanks for reaching out! I’m stoked to hear you’re checking out The Events Calendar and looking to go multisite. 🙂
You can get any license level to start and upgrade as you need to. Upgrading a license in the middle of a cycle will extend your license from the day you upgrade and reset the year from there. And the cost to upgrade is simply the price difference between licenses, so you won’t incur any additional charges beyond that.
Does this help answer your question? Please let me know.
Cheers!
Geoff