I might not understand your question completely, but;
1. In 2006, I completely converted one of my larger forum's sql tables collation to "utf8_general_ci" (UTF-8 Unicode) and never ran in to any problems - not even password problems. Then again, I never allowed the use of extended character sets to be accepted in user names or passwords, and those forums are 100% English, even though they cater mostly to just USA/CANADA/UK/AU/JAPAN
2. In 2010, as part making that same forum much more SEO friendly, I updated in the language files / header meta tags from "iso-8859-1" to "utf-8" @ Control Panel > Languages > Language Editor > generic.php > CHARSET
No problems whatsoever
