I don't really find much solid evidence to back this one up Serapy. Yes there are significant differences between the trial version and the final version but its also just a demo and so its not surprising that Square may have taken a few liberties to make it more exciting by changing things around. Roogle's post demonstrates this theory nicely.

Of anything, it looks like the Trial version could have been a playable Beta test given to fans as many factors of the gameplay were dramatically changed from the Trial version to the full release. Factors that had been criticized I might add (Renzouken trigger bar, draw amount, difficulty)

The major flaw in the theory is that Rinoa does not talk at all in the Trial version nor is her existence even recognized by other characters yet beyond that, all of the dialogue between Seifer, Zell and Squall remained the same as they are found in the full game. To me, this signifies that Rinoa was just there for the sake of the demo. It should also be noted, that I believe until the Trial version was released, the only characters confirmed were Squall and the "girl Squall is holding in the Amano logo". The Trial version was basically her first big reveal.

As for why the PAL demo is changed, its probably because the full versions had been released already in both Japan and the States. The Trial version had been released back when the game was still unfinished and being tested hence the Trial version had a very different battle system. Why then would you release a demo containing faulty gameplay that doesn't represent the actual game?