
Dear Heather Peake,
Thank you for reaching out. May I confirm if the Microsoft stipulation "never published site" you mentioned is "the publishing feature has never been activated", stated in below article?
Modernize your root site - SharePoint in Microsoft 365 | Microsoft Learn
If my understanding is correct, I would like to explain that this point is referring to "Publishing feature" on the site. You can check the feature by going to the site > site settings > site information > view all site settings > site collection features > SharePoint Server Publishing Infrastructure.
If the target site ever had this feature activated before, even it was deactivated later, it will not be allowed to use as root site.
And for newly created sites, this feature is disabled by default. So newly created communication site should meet the creteria to be swapped as root asite.
Let me know if it helps or if you need further explanation. Have a nice day!
Best Regards,
Community Moderator | Sophia