On the Modifications of Plot
Vaelyn

Reviewing the recent elements of the trailor, teaser images, et cetera, I think it is clear that Arwen replaces not Legolas in the Fellowship, but rather Glorfindel in the flight to the ford (I refer solely to the events, not the particular chapter).
I must admit that I think it's a little overboard to even change this, but, were they to change anything involving Arwen, this would be the best (and only) place in which to do it.

The character of Glorfindel has, as we know from Bakshi's 'Interpretation' (I use the term loosely!), been long picked on. He is, I would say, perhaps the only truly replaceable character.

One could claim that he was invaluable, being Sam's first glance at Elfdom--but this was in fact the role of Gildor Inglorion of the earlier chapters. Poor Gildor has most likely been removed along with Bombadil and Maggot. I should find it ironic if Maggot and Gildor were to remain and Bombadil still not be present. I would think it far better to remove Gildor and Maggot for screen time to show Bombadil. Of course, what PJ thinks in regard to the three, I have no clue.

As for Arwen being placed anywhere else beyond where she already has intruded, I would say to PJ, "Meddle not in the affairs of wizards, for they are subtle and quick to anger."

Also, though I'd rather not say this, I think Mark S. has taken a rather careless view of all these matters, and hasn't considered the true value of the originality in Tolkien's unedited works. Of course, I would sooner change the wording of the Bible than that of Tolkien.