Change instrument, undo does not undo sound

Steps to reproduce bug
1. Create score for piano
2. Add at least one note to the score
3. Right click on an empty part of the staff and choose "Staff Properties..."
4. "Change Instrument...." to electric piano, and click OK to dismiss dialogs
5. Edit > Undo
6. Play

Expected behavior: Sound during playback in step 6 should be acoustic piano

Actual behavior: Sound during playback is still electric piano

MuseScore version: 0.9.6.3 and nightly r.3623

(Operating System: Windows 7)

first reported by chen lung: http://musescore.org/en/node/7435