From Sonic Retro
Merging page histories
In some circumstances, Administrators are able merge page histories, in order to ensure that all edits to an article are reflected in a page's history.
For example, every non-minor article edit is credited to the Archivist who contributed it, available in the page history. However, if two articles contain the same or likened information that consensus requires be combined into one article, the only way for contributors to both articles to have their edits recorded in the new pages history is to merge the pages' histories.
Warning: this procedure may sometimes only be undone by a developer, spending quite silly amounts of time: to undo a merge, every single version has to be manually reassigned to the correct source page. Do not do this if you're not sure what you're doing.
Follow this procedure to merge page histories: