Changing the charset once running

Is it possibile to convert all existing posts and comments to another charset? I would like to convert my iso-8859-15 to utf-8, but I have 300 posts and 200 comments, and I don’t want to loose them or manually convert.

I need utf-8 because the press-it link now doesn’t work, it has charset problems (with utf-8 I don’t have this problem).

Thanks in advance.