When you are desperate and need to replace special characters across an entire wordpress database, here's one way of generating the right SQL statements to do that.

gistfile1.rb

terms = {
  'ë' => 'ë'
}
{
  'wp_posts' => %w{post_content post_title post_excerpt},
  'wp_comments' => %w{comment_author comment_author_url comment_content},
  'wp_terms' => %w{name},
  'wp_term_taxonomy' => %w{description}
}.each_pair do |table, columns|
  columns.each do |column|
    terms.each_pair do |search, replace|
      puts "UPDATE #{table} SET #{column} = REPLACE (#{column}, '#{search}', '#{replace}');"
    end
  end
end