You would need to engage Insert Mode (I) before you change the note durations.
Change durations only changes note durations!
Be aware, you will have problems if your passage includes tuplets. Also objects that are attached to rhythmic positions (time signatures, key signatures, text etc.) will not move.
Close! All the upstem notes did what I want. The downstem notes, which don’t appear in every measure, also had their durations halved, but they didn’t stay with their upstem notes.
This is to be expected. The first note of a voice will stay in it original place.
But you should be able to realign the additional voices (use the Filter to select the notes in each voice)