I realise I may be talking to myself here, but in case anyone is interested, I finally heard back from the iBookstore on this one. Yes really, I did. They said:
"For changes that are cosmetic we currently send push updates when there is an ePub not functioning properly but do not send push updates when an ePub is functioning as it should. We value your concern and will take this into consideration for future modifications.
The iBookstore Support Team"
So the fact that I have discovered how to make all the bridge books I've published look much nicer by being able to include unicode characters for the card symbols and made all the necessary changes and uploaded to the iBookstore still means all prior purchasers won't get any updated version unless they happen to know to delete their copy and redownload.
IMHO you can have as much technology as you want Apple but unless you have customer-centric processes to match you are failing.
Rant over. I may even give myself a correct answer.
I am also concerned about this issue.
Not only is it unfair to the customer, it can be unfair to the author/publisher, as well. I received feedback that some tables in my cycling skills book were not displaying on the iPad2. Since I don't have all three iPads and every Apple phone and other device, I couldn't check them all. The ePub looked fine in Aldiko on my Kindle Fire, so what can I do?
Anyway, I uploaded a new version through my distributor that should be working correctly, but there is no way to notify the reader to re-download the book. Since they happened to leave a comment, there is also no way to help them get a new copy and potentially update their review.
Something as simple as an "Updated" tag on the book in the iBooks library would be simple and useful.