As one who prefers the happy ending, I don't like the way the Animorphs ended. Many of them died, their internal characters changed, and dreams went unfulfilled. I find it very depressing to read the last few books despite highly enjoying the rest. What would you think of rewriting the ending to make it a happy ending? Save Rachel, put Jake and Cassie together, help Tobias reenter society, and don't end vaguely on some mission in space where our main characters seem to get captured or killed.
The ending does seem realistic, however. War does change people. If rewriting is a terrible idea, perhaps you would like to explain to me how the ending is actually good or what you like about it.