Hi, can anyone address this question?
Will downloading and installing avast free antivirus cancel out
avast internet security free trial and, hence, its automatic switch
to non-free internet security?
Thanks.
Simply uninstall Avast Internet Security and when asked, select to use Avast Free.
That’s the easiest way to switch from the paid product to the Free product. 
https://youtu.be/Etw0MRLCAF0
What you also have to consider is, to get the free trial of avast internet security, did you complete a payment instruction ?
For example when the trial runs out Avast would use that payment instruction to pay for the product. If you did then you would also have to cancel that payment instruction.