While you can technically revote for less weeks and this will become your new multiplier, your EGLs will still be locked for the original lock term because your EGLs are always locked for the longer term.
For example, if you originally voted for 8 weeks, 2 weeks pass and you revote (so 6 weeks are left) and pick 3 weeks, then your new multiplier will be 3 but your EGLs will still be locked for the 6 weeks left on your original vote. Thus, it is suggested that you always revote for at least as long as your EGLs are still locked.