When a musician is required to double on a different instrument for a section of a piece, the instruction to switch instruments is generally placed above the staff at the beginning of that section. A return to the primary instrument is handled in the same manner.
MuseScore enables users to insert a special class of text called Change Instrument text for this purpose. This class of text is different from either Staff or System text in that it links the text to the playback and changes the sound to the new instrument.
Version 2.1 introduces a greatly improved mid-staff instrument change over previous versions. There are still some limitations that need to be considered prior to using it.
Mid-staff instrument changes are limited to the same type of staff. For example, you cannot change between a percussion staff and a pitched instrument staff or vice versa.
The instrument name is not changed in the mixer. It will still be listed under the instrument in the original definition of the staff.
The key signature is not automatically updated at the instrument change. You must manually change the Key signature if needed.
You can now enter the notes a musician would play once the instrument is changed and the correct key signature is entered if necessary.
Unless you are changing the type of staff, you will always use the Change Instrument text.
There are several limitations to this in version 2.0 which should be understood before attempting to use it.
Automatic transposition from concert pitch to the appropriate key for the transposing brass and woodwind instruments is not currently supported. For changes to instruments notated in a different key (C flute to E♭ flute; Oboe to English Horn, etc.), the use of ordinary Staff Text to indicate the change is preferable, and the transposition must be done after the music is entered (using Notes→Transpose from the main menu). To avoid discord on playback, the instrument assigned to that staff should be muted in the F10 Mixer.
If it is necessary to hear the new instrument sound on playback, the Change Instrument text function must be used. However, after a mid-staff instrument change where the two instruments on the staff are not notated in the same key, no attempt should be made to enter new music directly from the keyboard. Instead, the music must be (a) pasted in, or (b) entered before the instrument change is affected. New input into measures following an instrument change is subject to two known program bugs, which cannot be resolved in the current 2.0.x versions without adversely affecting backwards compatibility. (This has been fixed in version 2.1) In addition, the score must remain notated in concert pitch, or discord will result from the transposition. As a convenience to the players, a copy of the part may be saved as a separate file and the required sections transposed to the appropriate key before the part is printed. (Note that transposing a linked part will affect the score as well.)
When changing from one concert-pitch instrument to another, or from one transposing instrument to another in the same key (Bb trumpet to Bb cornet or Flugelhorn, etc.), the Change Instrument Text may be used to ensure that the playback sound is altered to the new instrument. Input may be done in the usual manner, and is not affected by the bugs mentioned above.
There are some incompatibilities between the two versions.
Instrument changes created with version 2.0 and opened in version 2.1 or above will continue to either display the notes wrong or play the notes wrongs as in version 2.0. Deleting and reentering the instrument change will fix most incompatibility issues with only minor changes being needed.
Instrument changes created with version 2.1 or above and opened in version 2.0 will generally playback correctly but continue to display the wrong notes.
Right-click the text and choose "Change Instrument…"
Choose the instrument, then click OK