Default-Window-Style and Window-Style are two different things. If you want to change the WindowStyle, you should use that property. In addition take care for the handle of the figure!
%get handle:
myFigure= findobj('Tag','SomeUniqueTagHere') %make sure to get the right one
set(myFigure,'WindowStyle','normal')
I guess (but I'm not absolutely sure) default-properties will just be used during creation.