12 May 2012: Received a reply:
Hello Michi,
I believe there is a misunderstanding. To clarify, there are certain fields that you can make directly through iTunes Connect, while there are other changes that can only be made through the Contact Us form in iTunes Connect.
Changes you control through iTunes connect: updates to pricing, territory rights, Cleared For Sale/Not Cleared For Sale, sales start date, and DRM settings.
Please refer to the Publisher User Guide in iTunes Connect for more information.
Hmmm… I do wonder whether these people actually read the questions they are getting. Seeing that I asked about publication date, I would have appreciated getting information about changing the publication date, not about changing fields that I do not want to change…
After I checked, I found that it is not possible to update the publication date with iTunes Connect. The only thing that can be changed there is the sales start date, and trying that resulted in an error telling me that the sales start date cannot be in the past. Fair enough. It begs the question though why I was directed at iTunes Connect.
Then I tried with iTunes Producer. Things look more promising there. I followed the recommendations here:
iBooks Author: How to update your published book in the iBookstore
I searched for my book by ISBN, rather than opening the previously delivered package. That worked fine. In the metadata for the book, the publication date is editable, so I changed it to 4 May 2012.
While I was at it, I also updated the cover. Contrary to what Apple have said previously (namely, that the cover can only be updated by Apple), the KB article I linked to provides instructions for how to update the cover.
I left all other parts alone: no other metadata or content changes. The update went through and was accepted in a few seconds.
One glitch: the instructions for updating a cover require you to edit the .iba file and publish the updated cover by clicking File > Publish. I didn't do that and instead updated directly in iTunes Producer. This is somewhat strange, seeing that the first part of the article, which explains how to update metadate and book assets suggests to use iTunes Producer, but the second part suggests to use IBA. I expect that the two approaches should be equivalent, but one never knows…
Here is me with my fingers crossed, hoping that I haven't shot myself in the foot, having to watch my book disappear from the store…
Michi.