I don't see how her singing the song in a movie would prevent Taylor from hitting certain notes because the song is not sung on a theater stage. In fact since movie songs are recorded in a studio, I think the vocal performance has a chance to be that much better than live theater singing.
I think the one of the things most are concerned about is the songs being rearranged to make them easier for Taylor to "handle". Those breathy barely there pop/folk vocals which have given her a shockingly great pop music career simply aren't enough for the role she has undertaken. I hope, as do others, she has something in reserve. BTW, "Safe and Sound" did nothing to give me any more confidence in Taylor's vocal ability.
As Vanyel pointed out, the performers will be doing their songs LIVE, which is why I suggested that the filmmakers might be best served in staging On My Own using the slower and lower key of Fantine's Death/Come to Me in lieu of staging it traditionally (the two melodies are the same, but when it is used for On My Own, it's sped up considerably), because it would then minimize any issues that might arise should Swift be unable to hit the range of notes on On My Own exactly, but wouldn't significantly alter the song in any way.
I've noted this before, but I come from a very musically-inclined and vocally gifted family (both immediate and extended), and have personally seen two of my younger sisters sing songs that, on the surface, seem wrong for their voices - based on how they traditionally sing - and sing them flawlessly with very little to no alteration/re-arranging required. My sisters are both very talented, but they are nowhere nearly as talented as Taylor Swift, and it is for this reason that I personally believe that she'll be able to handle the role of Eponine without any problems whatsoever.
Having said that, Les Mis does have what I would consider to be a built-in 'safety mechanism' on the off-chance that Taylor can't hit the notes normally required for On My Own.
That 'safety mechanism' is the slower-keyed version of the OMO melody that is used for the song Fantine's Death (Come to Me). That slower-keyed melody is well within Taylor Swift's traditional vocal range.