When instrument is changed (via "Change Instrument" in Staff Properties"), initial clef is not changed (as seen in Instruments)
1. Create a new score with any instrument—say, electric guitar.
2. Go into Staff Properties, click the "Change Instrument..." button, and make it a different instrument—say, trombone.
3. Note that the clef has not changed.
4. Press [i], and see in the instruments dialog that the instrument is actually still electric guitar, not trombone, and still is assigned to treble clef 8vb, not bass clef. Apparently, nothing you can do will change this.