Hi @IT Guy
1) You can replace your root classic site with a modern one as seen in this article:
https://www.sharepointdiary.com/2019/08/sharepoint-online-change-root-site-to-modern.html
2) When you create the "POC" which I assume means Proof of Concept, it will have its own unique permissions set up for the site. Therefore, only you and who you have added to Administrators will have access to the site until you have shared it or added more members.
If this is helpful please accept answer.