Hello Kris,
First, your iCagenda installation on your Joomla 3 is "safe": it has no critical security issue (but your Joomla 3 installation is maybe not so safe as end of life...).
Only Joomla 6 from 6.0.0 to 6.1.1 has a critical security issue on attachment uploads (this is fixed in Joomla 6.1.2).
That means iCagenda running on all older versions prior to Joomla 6 (and all J6 version since 6.1.2) don't have a critical vulnerability.
The only common issue is the possibility to submit a not-approved event in quest access. The result is "fake" event added to the list of events, not visible on public frontend, but in admin, and with no possibility to exploit it.
So, more annoying than anything.
About your pro version, the version 3.6.6 is really old. If you don't have anymore an active pro subscription, so you can't update to pro.
If you want to install the free version over a pro version, then you will have first to remove the pro module (iC Event List) and the pro plugins, as it's using old code from 3.6.6 (December 2016... 10 years ago) may not run with component code from 2026...
This is like installing a core component of Joomla 6 on a Joomla 3 site, it will crash.
Another solution, if you want to keep the pro feature of iCagenda Pro, is to renew your subscription...
Best regards,
Cyril
