We report a new transition in the magnetic (H-T) phase diagram of the high-temperature superconductor YBa2Cu3O7. A sharp (T0.020 K) step in the irreversibility line is observed within about 1 K of Tc at fields considerably above the lower critical field. A contour map probing nonlinear region of the H-T plane reveals a backflow and reentrant behavior below the irreversibility line, reflecting the collapse step. These observations are consistent with the existence of a thermal softening boundary which crosses the irreversibility line in H-T plane, and at which vortex cores are delocalized on the scale of the coherence length. © 1991 The American Physical Society.