Rasputin - 5:12 pm on Jan 12, 2012 (gmt 0)

I've moved lots of sites from html to joomla, no particular problems and no loss in traffic, and no negative change in rankings as a result. And much easier to maintain once its setup.

Things that might cause difficulties:
- if you currently have filenames ending in .php that can be tricky at first to replicate in joomla
- if you have lots of pages in the root directory they take longer to setup because each one needs its own menu item to be declared in joomla (subdirectories you just define one menu item for the whole category)

There are joomla components that in theory can import a whole html site automatically but I've never used one.

But - I would wait for joomla 1.8 (out soon) if I was transferring a site, which will be the first 'full' version to include a proper 'one click upgrade' for new versions (like wordpress uses) which will take a lot of the pain out of using joomla.