Same ISBN but different edition and content?

[W]

A question in general, but I will provide an example to illustrate my point.
City of Bones by Cassandra Clare Paperback edition was re-published sometime in late 2012 - early 2013, with new additional content and a slightly modified cover under a different publisher (Margaret K. McElderry [originally published by Simon Pulse]), BUT under the same ISBN13 as the first paperback edition (Published in 2007).
The new paperback contains 2 previews, one of which is for a book that was first published in 2010.

The question is: Should a new "edition" be added to said book or should it simply remain the same? (Because the ISBN hasn't changed.)

[Paula]

A new alternate edition should be added. We do not change details on published books.

A note" Alternate Cover Edition:ISBN" should be added to the description and as a librarian note