[kh99]

If you create a plugin using hook bbcode_fetch_tags, you can change the html for those bbcodes. The replacement should be html with the text represented by %1$s. So you probably want something like:

$tag_list['no_option']['b']['html'] = '<span class="bold">%1$s</span>';
$tag_list['no_option']['i']['html'] = '<span class="italic">%1$s</span>';
$tag_list['no_option']['u']['html'] = '<span class="underline">%1$s</span>';