Similar to this question, but would like to know specifically if there's a way to do this through the iBookstore/iTunes Producer, and what the process might be.
The books are epub2 and epub3, not iBooks Author.
Yes, this happens automatically.
During the publishing process, you specify whether the book is new or an update to a previously published version. If it is an update, you must provide a new version number. Readers who downloaded your book are notified in iBooks that a new version is available.