The first distinction must be made here as to what qualifies as an "acceptable" change. The Lost World was written in 1912, and in certain unfortunate areas, betrays it's age. Certain racist attitudes prevalent in the book would have no place in today's world, and would prove to the detriment of any adaptation. To change this by, say, getting rid of the Gomez angle as well as Zambo altogether and making the war with the ape-men an issue of regretful necessity would give it more apt treatment. Then there are the dinosaurs. Bringing them in line with the current paradigm of how dinosaurs looked and behaved would not only be "permissible" (because I am the one who gives permission to these things of course=), but necessary. Another issue, a matter of simple logistics, is that the book would have to be edited for time and such. The full text would probably have to be contained within 4-6 cassettes, or an 8 hour TV miniseries if it were to be done practically verbatim. Obviously, while very cool, this would prove difficult. Editing for time is simply one of those things which we must live with.
Now, what about fundamental changes to the basic story? The most common is to add an element of romantic interest, such as Paula White in the 1925 movie, the changing of Professor Summerlee into a woman by Alien Voices, or the incorporation of two female leads in the TV series (though exactly who they are a love interest for, the male leads or the male audience, is left unclear=). Another recent commonality, as seen with the afformentioned womanizing of Summerlee or the sufferagette included in 1993's film adaptation, is the "liberated Victorian woman fighting the mores of her day". Then there is the "kiddizing" of the 1993 version (which was the epitome of everything that can go wrong with changing the original text until the 1998 version was made) by the inclusion of a cub reporter and the little indian girl he befriends; who succeed in saving the day of course, but not before having to be rescued by the adults several times over.
These changes may even be done with the best interests of the book at heart... Updating a classic science fiction work to be relevant in this day and age. The problem though, is that this is determining what The Lost World OUGHT to be, not what it IS. This is a littany of film makers and artists and producers and writers saying "well, if I wrote The Lost World, I would have done it this way...". The thing is, they DIDN'T write it... Sir Arthur Conan Doyle wrote it. It just so happened that Conan Doyle also knew what he was doing as well; The Lost World has survived for almost 90 years as one of the great classic works of science fiction. How many other works can truely make that claim? How many of the works comming out today will withstand that kind of test?
Science fiction today, the most consumeristic and ultimately forgettable at least, are formulatic at best. Have your big commando hero, a love interest, a few kids in peril, and you have a blockbuster movie. Does that sound familiar? That is exactly what has been happening to The Lost World: formulaization. "Well if Brand X dinosaur movie did this and made zillions of dollars, then doing the same thing with the title The Lost World will to!" This leads to the second most likely motive for changing the original novel.
Money and the desire to get it can be a very strong influence, and one that runs roughshod over creativity and artistic respect. Mutilating a classic piece of literature for the sake of a few bucks is not beneath more than a few people in Hollywood. Finally, some of the changes are just baffling and defeat any kind of logic or reasoning.
As we can and have seen, there are myriads of reasons why The Lost World is toyed with and tweaked and sometimes outright mutilated. In light of this one can't help but wonder if The Lost World will ever get the proper treatment it deserves. However, this may not be to totally discount it's adaptations. As mentioned before, the 1925 movie stands on it's own merit as a classic silent picture. The 1960 film may be considered good if you happen to like 50's and 60's dinosaur B-movies with big lizards in frills. The Alien Voices audio-dramatization has it's own merits as well. The Lost World adaptations seem to be one of those things which you just have to live with, and maybe look at with the eyes of a basic archetypal story with various good and not-so-good renditions.