It seems that the original WMD project is still accessible under http://code.google.com/p/wmd/, all links (including the download link) are dead however. So it might be better to change the links into http://web.archive.org/web/20110127184306/http://wmd-editor.com/ instead - it's still obvious that the link goes to a dead project but at least that page has more information and the download works.
I don't think that pointing the links to a different project is justified, even if it is equivalent. If I were reviewing an edit like that I would likely consider it a radical change:
This edit changes too much in the original post; the original meaning or intent of the post would be lost.
Edit: In the light of the answer by Sha Dow Wiz Ard, a more differentiated approach might be a better idea. When I look at the posts mentioning WMD editor I see the following types:
- Questions about issues with the WMD editor: Here the link here should definitely be changed into a Web Archive link, these questions have no relation to the fork. Further, maybe these questions need to be closed ("unlikely to help future visitors").
- Answers mentioning WMD as the editor used by Stack Overflow: These can be updated with a link to PageDown, that's what Stack Overflow is using now.
- Answers recommending several editors: Removing the part about WMD sounds like the best solution given that the project is dead and its downloads unavailable. Alternatively one could remove the link and add a note:
(Update: this project is dead and no longer an option)