Variations in the recorded magnetization structure have been investigated using a magnetic force microscope (MFM) for a perpendicular recording medium when exposed to perpendicular magnetic fields. The recorded bit shape starts to change from around the magnetic field several percent lower than the reverse-domain nucleation field, H_n. The bit shape variations can be classified into four manners, where the magnetization reversal takes place from the bit edge (two types) and from the inside (two types). We also found that bit-shape variations progress preferentially near the recorded track edges rather than along the track center. A useful technique of observing the same area for repeated load-unload cycles of samples with MFM is also reported.