Migrate a standalone WordPress site into a new multisite network

I run a standalone WordPress site for [clientname.com] that needs to become the first
site (blog ID 1) in a new WordPress multisite network I'm setting up for [agency name],
which will eventually host [3-5] more client sites under subdirectories like
network.com/clientb.

Plan this conversion so the existing site's content, users, and SEO survive the
restructuring intact, reasoning through what a "how to enable multisite" tutorial
usually skips:

1. Table prefix and data integrity: explain what actually happens to the existing
   wp_ prefixed tables when multisite converts this site to blog ID 1 (they stay as
   wp_, only additional sites get wp_2_, wp_3_ etc.), and where that assumption breaks
   if any plugin or custom code has hardcoded table names.
2. Media path shift: media uploaded before conversion lives at
   wp-content/uploads/2026/06/file.jpg, but new uploads after conversion move to
   wp-content/uploads/sites/1/2026/07/file.jpg. Tell me how to avoid broken image URLs
   in old posts (existing serialized content still points to the pre-multisite path,
   which must keep resolving) while getting new uploads onto the correct multisite
   path.
3. Plugin and permission fallout: which currently-active plugins are likely to break or
   need network activation instead of per-site activation, how capabilities change for
   existing non-admin users once they're in a network context, and what to check in
   .htaccess since multisite requires different rewrite rules for subdirectory
   installs.
4. Rollback plan: since this conversion is difficult to reverse cleanly, describe the
   specific backup (database plus wp-content) I need before running the conversion,
   and how to verify it's restorable, not just present.

Flag anything here that would only surface once I try adding the second site to the
network, so I catch it during this first conversion instead of later.