In any given (Western) key, the major and minor scales have most of their notes in common. So, transposition between major and minor does not result in a 'night

In any given (Western) key, the major and minor scales have most of their notes in common. So, transposition between major and minor does not result in a 'night and day' difference - depending on the melody, there may be only a few points at which different notes are used. These changes, however, are enough to alter the overall mood of the music for a typical listener - e.g. in the case of the Imperial March, from menacing to upbeat.