Not being able to convert a classic site to a modern site is the reason we still run classic sites to this day despite moving 100% of our environment to Office 365 five years ago. Enabling modern pages on a classic site is incomplete. We have painfully migrated some sites to modern, but our root home page for our tenant, the most popular page, remains an OLD classic SharePoint page. Thus, modern sites are useless to us until we get our home page converted. We are still waiting for the convert to communication site PS command that was announce last year. This concept of swapping out the root site of a tenant is perfect! However, "Coming Soon" means nothing to me and actually irritates me. We've been in a coming soon state since Modern Pages was announced. Until I can convert the root of my tenant, modern pages remains out of reach for us.