This is exactly what I predicted in an older post.

It's not very important for big fans, they'll find the last book no matter what and often follow WOT news.

For the "masses" they needed to rebuild their "brand" if they had no Sweet painting to use for the last book, and the best way to do this is to introduce first the earlier books with new covers, as their titles are well known, and leave time for bookstore goers to become familiar with the new design before the HC of AMOL comes out. Fan reception has been pretty good for the e book covers, and quite a bit of effort was involved from Tor and Harriet to create them, so it's no surprise they chose to use that art to redesign the books.

They seem to make a bigger deal with the release of the three first books, which is quite logical considering their intent. Normally, if Tor follows what it usually does when it switches design for ongoing series (eg: Malazan), the rest of the series will appear with the new covers out of publication order, as the stock for each title runs out through the year (which doesn't mean they won't "officially" set a release date for each title to be able to have more WOT news for the fans in the year where they'll want our attention, but in practice the books would normally appear in stores and at distribution centers only when the stocks of the old ones are out.. it could be quite some time for the later series, which sells a lot less tan the early books).

I really doubt they'll delay the releases beyond next fall, personally. Thing is, they'll almost certainly want to have their nice displays with all the PB with the new cover art to put out come november, next to the last of the hardcovers. It's how they most likely mean to attract attention of store goers who follow WOT but casually (or who might buy the book as a gift for others), and who might miss the new release in store if they see only the HC without the usual WOT look.

About the new art... most of it was pretty good, IRRC, and I can't say I'll shed a tear to see the horrific Sweet covers vanish for good in the near future. The layout is fine. It's a vast improvement over the tacky old layout. This actually look like real books!