No idea, I just know for example when you sell through Amazon the Product ID always points to the same book. When it gets updated, you have a link on Manage Your Kindle saying there is an update available, and when you click it, references to downloading the book will I guess yield whatever version their internal records say you have.
They probably
do have to keep track of different versions, if only because they give you the option of automatic or manual updates, so some people may retain the older version.
I imagine this is the type of thing best managed by the seller's website under Customer Accounts (i.e. always yield a .acsm for the latest version, and notify the user via email or otherwise if they should redownload the book). I believe the .acsm is of time-limited value anyway, so the whole process should be transparent to the user.
The only way it could be easier is if you could automatically push the book to their devices, but you'd need your own client instead of ADE for that, and then you might find some users unhappy and complaining about Orwellian behavior.
