May be a dumb question but I would like to know if it is possible to revamp a site that uses xoops and turn it into a plain html site.
Sure, just download the site with one of the "offline browsers" that save the entire website to your harddrive. I don't know what you gain by doing it, but it can certainly be done.
Why do you want to switch from cms to html. CMS will keep your work easier. If you think you don't get indexed by SE then you could use mor rewrite to convert dynamic pages. Just go to google and search for it
As exam said, it can be done easily. If you need help, doing it, pm me the URL and I will look at it and suggest.
I think he might want to use CMS to create the site. Then when he's satisfied and wants it finalized, he can turn it into a static html site. That's what I'd do. I'd then update the static html from time to time using the CMS. Maybe ...