Upgrading a WP site to WP Multisite...Can you do this?

I have a standard WP install that I did via FTP on my server.

My site is live with content but I really need a WP Multisite configuration for others projects.

Does anyone know if I can reconfigure for a subdomain WP Multisite install without any issues.

Or do I have to start from scratch?

Thanks,
Randal