If you have for example 3 or 4 quarter notes already in a measure and want to 
change one of them to a half note, you're going to have to delete at least one
of those quarter notes directly after that half note. 

The easiest thing to do is to erase the notes after the note you want to 
change to a half. Then change it to a half note and put in the rest of the 
notes.

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