change "head scheme" depending on note pitch

Here's one way:
Let's assume you wish to apply pitch names to all notes above C5 (the middle space on the treble staff).

If there seem to be fewer notes above C5 than below, select those notes above C5 as follows::
Right click on a D5 -> Select -> More... -> then put a check in the 'Same pitch' box (shows D5), click OK.
Right click on a E5 -> Select -> More... -> then put a check in the 'Same pitch' box (shows E5), click 'Add to selection' button, then OK.
Keep repeating as needed for any other notes above C5. When they are all added to the selection, change the head scheme in the Inspector.

If there seem to be fewer notes below C5 than above:
In this case, to save keystrokes, you can first select all notes then subtract those fewer ("below") notes from the selection, leaving you with only the notes above C5. Then change the pitch names of the resulting ("above") note selection.

You could:
A.) write a plugin

B.) do the manual task as posted by Jm6stringer above

C.) sneaky workaround to make the manual task possibly easier, depending on how many different pitches you need to select otherwise:

1. Right-click the staff, choose "split staff" and set your split point.
2. Select the top/bottom staff as desired
3. Change the noteheads using the inspector
4. Select both staves
5. Tools → Implode then press voice 1 button
6. Select top staff
7. Tools → Implode again
8. Edit → Instruments… and remove the additional staff