As the author says, this WILL WORK.  But the motor speed will no longer be regulated, as it is originally.  You'll find yourself constantly correcting the speed, not to mention anomalous Rew and FF.  You'll probably have wow and flutter you can't fix.  And in the occasional machine with electronic motion-sensing auto shut-off, it will auto shut-off at slow speed.  This makes me think I should start selling adjustable speed motors!  (I'm not kidding.)