Daniel, there have never, ever, been any changes in ikiwiki can could result in GUID changes on upgrade, with the exception of enabling usedirs, which is something you have to do manually. And there never will be any if I have anything to say about it..

Of course, you can cause the GUIDs of posts to change by renaming or moving the files around, or by changing the url to your blog.

Doesn't explain why it happened of course..

PS, can I git clone your blog from somewhere? There's an interesting blip in your RecentChanges page that I'd like to figure out, where one time the change author is listed as "Daniel Burrows Daniel_Burrows".