  Solar coronal mass ejections are the most energetic events in the Solar
System. In their standard formation model, a magnetic flux rope builds up into
a coronal mass ejection through magnetic reconnection that continually converts
overlying, untwisted magnetic flux into twisted flux enveloping the
pre-existing rope. However, only a minority of coronal mass ejections carry a
coherent magnetic flux rope as their core structure, which casts doubt on the
universality of this orderly wrapping process. Here we provide observational
evidence of a different formation and eruption mechanism of a magnetic flux
rope from an S-shaped thread, where its magnetic flux is fully replaced via
flare reconnections. One of the footpoints of the sigmoidal feature slipped and
expanded during the formation, and then moved to a completely new place,
associated with the highly dynamical evolution of flare ribbons and a twofold
increase in magnetic flux through the footpoint, during the eruption. Such a
configuration is not predicted by standard formation models or numerical
simulations and highlights the three-dimensional nature of magnetic
reconnections between the flux rope and the surrounding magnetic field.
