Transposition is simple: C in C major reads as G in G major, but if that very C is C4, what G (G4 or G5) is the corresponding G? What is the rule to figure out the octave?
This is impossible to answer, up-transpose is as legitimate as down-transpose. It completely depends on the purpose of the transposition. For a singer the adjustment may be necessary to match his or her vocal range. It then depends, whether the high notes were the problematic ones or the low ones.
The only simple cases are adjusting a given score for a transposing instrument: then the diffrerence between the sounding note and the written one is clearly defined.