I don't really believe Ali in "The Brave Longneck Scheme" has been changed all that much. Based on what I recall of her in LBT4, her portrayal seems pretty accurate.
In the movie, she was quite shy and refused to interact with Littlefoot's friends; the only reason she ever did in the first place was because Littlefoot was in danger and they were the only ones who could help. Had Rhett been in the herd at the time, she'd probably go to him first, since he's of the same species as her.
As for her being gullible, which I think was brought up before, I attribute this to the fact that Rhett is another member of her species. Ali has spent most of her life with another longnecks; the only time we know for sure she has been with different species was the events of LBT4. Ali trusts her own kind more than other species, so she probably never saw a reason why Rhett would lie to her. If Rhett were another species of dinosaur, Ali probably wouldn't have been so eager to believe him.
Ali hasn't been in the valley since LBT4 and, as far as we know, her only other visit besides that is "The Brave Longneck Scheme". There is a considerable time span between these two events. While an exact time isn't known, considering evidence provided by "Star Day Celebration", it has been at least a year, probably longer than that given the other movies. And Ali has only been in the valley for a short time. And in that time, we don't know for sure if she has encountered other species like she had in the Great Valley; it may have been that she, like before, again only encountered other longnecks, and possibly started to trust longnecks a lot more than other species again.
True, Littlefoot is a longneck and Ali could probably trust him. But to point out, Ali has only been friends with him for a short time, and his friends even shorter since she was afraid of them earlier. But with Rhett, it's likely she knew him for a much longer time. While one can't place exactly when Rhett showed up, it seems possible, at least for me, that Rhett has been in the herd long enough for Ali to be willing to trust him over Littlefoot and the others, whom, as I pointed out, she had only known and interacted with for a short time in the fourth movie.