A fairly comprehensive acts-as-taggable-on tag migrator.

tag_migrator.rb

class TagMigrator
  class << self

    def list_tags(search_term = nil)
      puts Tag.where('name like ?', "%#{search_term}%").order(:name).map(&:name)
    end

    def migrate!(old_name, new_name)
      if old_name.downcase == new_name.downcase
        rename!(old_name, new_name)
        return
      end

      old_tag = Tag.find_by_name!(old_name)
      new_tag = Tag.find_or_create_by_name(new_name)

      execute("update taggings set tag_id = #{new_tag.id} where tag_id = #{old_tag.id}")
      old_tag.destroy
    end

    def rename!(old_name, new_name)
      raise_if_about_to_clobber_another_tag(old_name, new_name)

      tag = Tag.find_by_name!(old_name)
      tag.update_column(:name, new_name)
    end

    def remove!(tag_name)
      tag_to_remove = Tag.where(:name => tag_name).first

      execute("delete from taggings where tag_id = #{tag_to_remove.id}")
      tag_to_remove.destroy
    end

    private
    def execute(sql)
      ActiveRecord::Base.connection.execute(sql)
    end

    def raise_if_about_to_clobber_another_tag(old_name, new_name)
      if (t = Tag.find_by_name(new_name)) && t.name.downcase != old_name.downcase
        raise MigrationError.new('Cannot rename to a tag that already exists. Use migrate! instead.')
      end
    end

    class ::MigrationError < StandardError; end

  end
end