The line in FES looks like a typical synchronization sign similar to those we can see in this Nocturne a dozen or so times, e.g. in bars 35-36. Yet, the meaning of the line here is different: it changes significantly the sense of the rhythm of the right-hand part. In this situation it is strange that other signs confirming the intention of such a change are missing, e.g. the third quintuplet beam.
It should also be remembered that Chopin's teaching copies sometimes contain additions with unclear meaning e.g. characteristic slanted crosses or lines of various length. Those signs were probably written into the score to mark some important verbal instructions of the composer.
In the editors' opinion, one should be cautious when considering the possible rhythmical variant here.
Compare the passage in the sources »
category imprint: Interpretations within context; Differences between sources
issues: Annotations in teaching copies, Synchronization markings, Annotations in FES, Authentic post-publication changes and variants
notation: Rhythm