Switching between straight and swing feel mid-piece

I’ve had success changing the feel of an entire composition from straight to swing and back by using the ‘master track’ menu option.  It looks as if there is a way to change mid-piece by entering ‘swing’ or ‘straight’ as an expression marking.  ‘Playback’ is even a menu option when selecting ‘swing’ in the score.  Selecting playback allows me to choose the specific flavor of swing i want.  But then... no change at all in the playback.  Please help.  All around this is a wonderful app.  I recommend it highly to my students and colleagues.