How to change enharmonic key signatures for transposing instruments
There are a few limitations to using these instructions. One limitation is that theoretical key signatures that contain double sharps or double flats are not supported by MuseScore, so for example: changing the key from A♭ to G♯ is impossible using this method and you cannot get proper playback. Another limitation is that transposition of custom key signatures does not work.
Select the section of music with the wrong key signature: this may be the entire staff or only a few measures in the staff.
From the menu bar, select Notes→Transpose... Select the closest key below the concert key signature that contains sharps when transposed. For example Concert B♭ transposes to the Key of G on an E♭ saxophone.
Make sure that you have checked "By Key", selected "Down" in the "By key" section, and checked "Transpose Key Signature" (near the middle of the dialog box). Note: this will not be the final key signature but will have sharps or flats like you want in your final key.
Make sure that the same measures are selected.
You now need to do some counting to determine how far you transposed the section of music. It is best to count how many half steps it was transposed. For example transposing from the key of E♭ to the Key of B would transpose the note 4 half steps.
Open the Notes→Transpose... dialog again. Check "By Interval"; select "Up" in the same section; and make sure that "Transpose key signatures" is checked.
In the drop-down menu in this section, select the interval that is needed to return the notes to their original pitch.
Press OK. The key signature will now have the correct number of sharps or flats and the notes will be at the correct pitch.