Good afternoon Rob and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
Do I need to downgrade Events Calendar 4.5.1 to a previous version to be compatible with Events Calendar Pro 3.12.6
The short answer is yes. You have to use “matching versions” (at least as far as publication date is concerned) for things to run smoothly.
I recommend reading the following: https://theeventscalendar.com/knowledgebase/downgrading-plugin-past-version/
You could snag a copy of the Events Calendar 3.12.6 here https://wordpress.org/plugins/the-events-calendar/advanced/
With that in mind, 3.12.6 is almost 2 years old. As you can imagine, we simply cannot guarantee it will work properly with the latest version of WordPress, but it is possible it will still do.
Let me know how that goes.
Best regards,
Geoff B.