Barnes & Noble hasn't made any formal announcement about the change, but a reader has tipped me to a week-old post on B&N's forums where B&N announced the news:
We've recently made some updates to NOOK Press to improve your self-publishing experience!
Now you can edit your Manuscripts that are already on sale as NOOK Books -- without creating a new Project or removing it from sale.
To start using NOOK Press, go to www.nookpress.com and click “Get Started”. Create a NOOK Press account and easily sync your PubIt! account to transfer all of your PubIt! account, title, and sales information.
Barnes & Noble never made it clear why this was left out, but more than a few authors have probably been affected by its absence. If nothing else, the authors probably noticed the missing feature when they tried to correct typos and other errors.
Editing older ebooks is also going to prove usef ul to authors who have to edit time sensitive ebooks. Jason Matthews, the reader who pointed out this issue last month, writes how-to guides for self-publishing an ebook. His ebooks need to be refreshed every few months with the latest info on the publishing industry, and now he will be able to do so.
B&N has also confirmed the new feature in the Nook Press FAQ.