It's not so bad if the change is suitable such as CPT's The Doping Affair becoming The Pony Dopers, which made sure the audience was aware it was a child's pony book. Dream of Fair Horses and it's American counterpart The Fields Of Praise are okay if you have actually read the book, on the other hand if you have you probably won't part with it for 'a quarter horse' (or your equivalent) so wouldn't be looking anyway. I have just found out Janet Young Rider aka Horse For The Holidays is also aka Last Summer To Ride, which I was rather pleased to find out about as it was in my shopping basket to purchase tonight! What either subsequent title has to do with the first is beyond me? I was also caught out by the Kestrels/Horseshoe's series. I imported the Horseshoes from USA to find I already had them as Kestrels. I should have known really as some have same title and some have very subtle differences like The Perfect horse/The Special Pony and Jumping lessons/Pony To Jump.
I admit to buying several editions of the same book, sometimes, because I like all of them and can't decide which one to have but it's so infuriating when you've spent rather too much on one to find you've had it years (and aren't that fussed about it to boot!).
Saying all this one of my favourite books is K M Peyton's Free Rein/Last Ditch. I spent far too much buying Free Rein, as I loved the cover, knowing full it was the same book I had spent a small fortune the year before as Last Ditch. But at least I knew and it wasn't a surprise. Not all suprises are good.