I am working with a colleague to update his old, static, commercial HTML site. He started it in 1999, and has worked on it steadily, so it currently has hundreds of pages. These are divided between a root domain (which has the most content) and 11 subdomains.
I've enabled multisite (subfolders) on my development site before, and am comfy with the process and network admin panel. I've also worked out the process of transferring the HTML content and preserving (in most cases) the URLs. Also got a handle on the theme and most of the other details... except one.
As the title states, it's the replacement process that's problematic. He's not changing hosts or domains, so we'll be replacing a live site in-situ, with an eye toward minimizing any downtime. I've seen the process explained well for static-to-WP upgrades on a single domain/site; but I haven't seen a good explanation for static "multisite" to WP-multisite.
Would it be best to treat this as if this was a single domain/site transfer, by transferring the root domain, then enabling multisite and adding/transferring the subdomains, one at a time? Three of the smaller subdomains are already on (older versions) of WP, so I expect we'll have to do this anyway, at least in part.