Can an existing Module be turned into a weighted Module?
Modules don't have any points directly associated with them and thus cannot be weighted in Canvas.
Assignment groups can be weighted. You can put all of your items from a module into an assignment group and weight them that way. You can even place items from the same assignment group into different modules (for example, I have an assignment group for exams, but exams are spread out across several modules).
If you choose to use weighted assignment groups, you must make all assignment groups weighted -- you cannot have some points based and some weighted. But yes, you can change from a non-weighted system to a weighted-system. If the course has already began, you should be careful not to mess up the gradebook and what is stated in the syllabus when you switch.
This lesson from the Canvas Instructor Guide may be helpful: How do I weight the final course grade based on assignment groups?
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