Convert to WordPress

What might go wrong if we were to set up WordPress with Fatastico right alongside (that is, directly under /public_html) an existing HTML/CSS site? Our guess is that, if someone clicks on our URL, its current index.htm would be found first, so the original site could co-exist peacefully while we migrate text into posts — and ultimately kill index.htm so that it would go down the WordPress hierarchy.

But it’s pretty hazardous to guess. The Fantastico setup may simply take over. Anyone been down this road?