(bibtex-beginning-of-entry)
(re-search-forward bibtex-entry-maybe-empty-head)
(if (match-beginning bibtex-key-in-head)
(delete-region (match-beginning bibtex-key-in-head)
(match-end bibtex-key-in-head)))
(insert new-key)
(bibtex-beginning-of-entry)
(re-search-forward bibtex-entry-maybe-empty-head)
(if (match-beginning bibtex-key-in-head)
(delete-region (match-beginning bibtex-key-in-head)
(match-end bibtex-key-in-head)))
(insert new-key)