I think the implicit message of this correcting e-mail from Caitlin Hopkins is that the Atlantic, of all magazines, shouldn't mess with Longfellow:
Please note that "I Heard the Bells on Christmas Day" a.k.a. "Christmas Bells" was written by Henry Wadsworth Longfellow in 1864, and not, as you state on your blog, by Johnny Marks.
The poem was set to music by John B. Calkin in 1872 and re-arranged by Johnny Marks in the 1950s. Marks' version is sometimes said to be "based on" Longfellow's poem, but the only difference between the poem and the carol (other than the title) is the omission of two stanzas about the Civil War (though the poem makes a whole lot more sense if you leave those stanzas in).