You do indeed need to change it from all those wikis. It is typically something that a bot does well, so the best thing to do (if there are more than a handful of languages involved) is probably to contact a bot owner and let them do it. I have taken this message as such a request, and resolved the Ferenc Farkas issue.
However, whether done by bot or manually, there is still the problem that there might be a bot that sees some of the languages before, and others after the change; such a bot would consider the interwiki a 'missing' one on the languages coming afterward, and re-add it. I don't know of an easy way to resolve this, the only thing I can think of is adding an article on the composer on eo:, or on the esperantist on one of the other languages. Or keep an eye at the articles, and keep changing until we get lucky...
There is a method keeping bots away from putting a wrong iw to a single page by commenting out the wrong link like <!-- [[de:Bad Link]] --> But this does not prevent the bots from putting on others sites pages.
Regards xqt