default transpose is not useful
S5 - Suggestion
copy alto part to tenor or tenor to soprano etc
the copied part is one octave to high (or too low)
select transpose tool, transpose by interval, down or up
default interval transposition is unison
suggest reversing that to make default transposition one octave
The easiest way to transpose an entire part an octave up or down is not the transpose tool, but selecting the entire staff, and ctrl-down (or ctrl-up, Cmd on Mac). Ctrl-Home, shift-Ctrl-End to select a whole staff. I would further argue that copying an alto part to tenor at the unison or octave is not correct (your tenors will not be able to get the low notes at the octave, or high notes at the unison).
Also, the default transposition of unison is harmless if you decide you don't want any transposition and press enter. ctrl+arrow is a much easier way to transpose an octave.
Thanks for the tips
Octave transposing is not a better default than unison (AKA none). Alone the question in what direction should it be, up or down, demonstrates this. Basically there is no good default, not that would be right in more than 50% of the uses cases, noe that would be very wrong in more than 50%.
Hence the default of no transpositioning is the only sane and safe default there is, it at least doesn't do any harm.
What might be helpfull there though is to bring up a tour explaining the options, esp. in case of unison transpositioning.