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.
Slowing the song down and lowering the key in which it is played likely won't help the apparent lack of strength in Taylor's voice. This is what I was referring to when I mentioned the "breathy, barely there pop/folk" vocals. I just watched part of a concert of hers on Palladia. She was doing one of her own songs with an uptempo kind of loud arrangement. Her voice was nearly overwhelmed by her own band.
I think she'll likely hit the notes okay, or find some adequate (but perhaps less satisfying) alternatives, but the songs she will be singing were originally performed by singers with strong robust voices and I'm afraid compared to them, Taylor's performance will pale.
But on the other hand, when Beyonce was singing with Destiny's Child, I didn't think much of her vocal ability, at least the stuff by them I heard on the radio. But since she has been out on her own, singing different types of songs, it's pretty obvious she was hiding a pretty strong instrument. I'm hoping this turns out to be the case with Taylor.