While at Barnes & Noble this afternoon hoping to scope out the new Eulalia! paperbacks, I spotted the Ace 10th Anniversary edition of Redwall, however it is now re-packaged as the 20th Anniversary edition.
The book is identical cover to cover; no chapter icons, the introduction is exactly the same, etc. Even the ISBN, ISBN 9780441005482 is the same.
The only thing different is the "20th Anniversary" text. Given that the book has been re-issued in this way, I highly doubt that a 20th Anniversary paperback with Troy Howell's new art will be available.
My Barnes & Noble did not have the Eulalia! paperbacks, however that does not mean that yours will not. My closest B&N is actually notoriously bad in terms of stocking Brian Jacques material.
If you've been waiting for that paperback release, now's your chance, it was scheduled for publication yesterday.