Here are examples of one title change that worked for me, and two that didn't.
Karen Harper, Passion's Reign (1983) vs. The Last Boleyn (2006)
Harper's novel about Mary Boleyn, written and published well over a decade before Philippa Gregory's take on the subject, appeared in 1983 as a paperback romance. It was re-released last month as a mainstream historical by Three Rivers Press in trade paperback, because of Harper's popularity with historical fans (due to her Queen Elizabeth I series) and the literary craze with all things Tudor. The publisher acknowledges the title change on the cover, which helps, and it's fair to say that mainstream historical fiction fans would have a low tolerance for the earlier title, regardless of the novel's quality. Wish I could find my copy to scan in the earlier cover, because it's very typical for a 1980s historical romance. To give you some indication, the cover of her 1984 novel Sweet Passion's Pain has her heroine Joan of Kent wearing dark blue eyeliner, bright pink lip gloss, and magenta nail polish. Anyway, in my book this title change is a plus, though I haven't read the novel in a while, and don't know how well the prose has held up over time.
Melvyn Bragg, Credo (UK) vs. The Sword and the Miracle (US)
I read this based on the British edition, and I loved it - I also knew, based on the cover and the title, exactly what I would be getting. No doubt many of you have already read this epic biographical novel of St. Bega and the struggle between Celtic and Roman Christianity in early medieval Britain. I can imagine the American publisher thought that keeping the title as Credo in the States would either result in readers asking "huh, what's a credo?" (which may not be too far off) or bookstores mistakenly stocking it in Religion. But the title of The Sword and the Miracle, while evocative in its own pseudo-Arthurian way, is just plain deceptive. It implies people will be getting a historical fantasy that may have something to do with the Holy Grail. And then there's the dust jacket. One Amazon reviewer called it right: "Fabulous [book] - but what's Fabio doing on the cover?"
Anne Perry, Slaves and Obsession (UK) vs. Slaves of Obsession (US)
This is the novel in the Monk series that's set partly during the US Civil War, and partly in Victorian London. Funny how changing one little word can change the meaning of the title completely. To the American audience, the title has nothing to do with slavery and everything to do with obsession. This makes it sound more like a Judith Krantz novel or the 1860s version of some glitzy family saga. Maybe the publisher felt that mentioning (or implying) anything about slavery in an Anne Perry title would turn off American readers, especially since her Monk series is more popular in the US than abroad. If nothing else, the two titles are similar enough that readers won't be confused - apart from wondering why it was changed in the first place.
More examples to come.