Even though you may have Assignments in the Grade Book, all the grades etc haven't changed you are only changing the equation to get your average. So if you did total points, and then decided you wanted Total Points with weights,
that's not much of a shift and only requires a slight adjustment to the actual equation.

There is ONE you have to be careful with changing, and that is switching from a Total Points method to a Percentage method. 

To change your Gradebook Calculations:
  1. Click Classes
  2. Select the Class you are looking to edit the calculations for
  3. Click the Gradebook tab
  4. Click Setup (Gear Icon)
  5. Select the Calculation method you want to use from the Calculation method dropdown
  6. Click Save or Save and Close