Good evening Tom and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
Actually, Event Tickets Plus is built on top of Wootickets. There were no feature loss in the process (only new features).
When you are ready to migrate, you can simply follow this guide: https://theeventscalendar.com/knowledgebase/moving-to-event-tickets-plus/
You are right about the Wootickets key not being compatible. However, not having the key will not prevent the plugin from working properly. The only thing it prevents is updating (which should not be a problem).
So in short, you can totally go without the license key.
If you don’t have the files, then that could be arranged too.
Let me know if that helps.
Have a great day!
Geoff B.