A number of authors have written me recently about self publishing a book they've written for another publisher. Whether that other publisher is a trade press or a subsidy press, the one thing that remains constant is that the book contract defines what you can and cannot do without inviting legal action. It's not who wrote the book or who filed the copyright that defines who has the right to publish the work, it's the contract. In most cases, book contracts do give authors the ability to regain the rights to a work under certain conditions, ranging from simple notification for an ethical subsidy press, to being declared out-of-print and approval of a written request for many trades.
The next question that comes up from authors who have regained the rights to their work is, "Do I have to change the title?" Publishers buy and sell the rights to books all of the time, and especially when it comes to the classics, changing the title is out of the question. But to reduce confusion for the reading public and book collectors, an edition name or number is often added, sometimes as a subtitle. I'm a fan of doing this just so a reader who may own or have read a previous edition won't be disappointed when the book they receive in the mail doesn't exactly match what they expected. There may also be trademark issues involved, and it's unlikely the author would gain rights to cover art that was paid for by the publisher. If you haven't reached an iron-clad agreement with the previous publisher as to what you can and can't do with the new edition, I'd strongly getting legal advice from a publishing attorney.
Another question that comes up quite a bit is whether a publisher is required to create a new edition with a new ISBN number when making minor updates to a book. There are no laws I'm aware of governing how publishers must manage edition numbers, and it's certainly the norm to do minor corrections to a book without releasing a new edition. Major corrections or edits may result in a revised edition, and updates are generally given new edition numbers. Generally speaking, publishers welcome the chance to update nonfiction works and create a whole new edition with a new ISBN number, as it raises the possibility of repeat sales. It's not necessary to release a new edition just to change the price
I'm somewhat on the fence when it comes to making substantial changes to books to keep them up-to-date without publishing a new edition for marketing reasons, especially as relates to Amazon. I understand the argument of the authors of published books who want to keep them more current than is practical with releasing new editions, even with print-on-demand, but it does create problem for buyers of the second hand books which account for a large chunk of Amazon sales. If a potential buyer reads a review praising a book for its up-to-date instructions for doing X, but there's another version of the book available with the same ISBN number, cover design, etc, that has out-of-date instructions for doing X, it's a problem.
One potential solution would be to use the author tools on Amazon, such as the blog, to try to keep potential buyers informed of updates and to advise them against buying used copies for that reason. I don't know where Amazon would stand on that approach. Another possibility would be to date stamp the book in the cover design or through a subtitle change. While this might raise some eyebrows at publishers locked into large offset runs, the world does move forward from time to time, and the combination of print-on-demand technology with computer cataloging systems certainly makes it practical. At least for online sales, inserting "Updated 11/07" at the start of the subtitle is probably the best bet, and if it became standardized, we could probably drop the "updated" and just go with the month/year. Unfortunately, the current ISBN centric system would lead to used copies be listed with the new subtitle, but at least the potential buyer would be forewarned. If the date code was added to the cover design in a little white circle on the spine or cover, it would serve for bookstore books as well. Of course, publishers with serious bookstore stocking would avoid such intra-edition updates at all costs for fear of getting swamped in returns. Just another reason to consider print-on-demand:-)