It is not possible to make timesheet changes comment mandatory without having DCAA turned ON.

This setting is located on the DCAA Compliance tab of the System Preferences page in Web TimeSheet. Once it's enabled, it'll ensure that all changes made to a user's timesheet are documented. For timesheets, change comments are required for all changes made to the timesheet after submission, as well as for any changes made by a substitute user before submission. If you turn off this setting, Web TimeSheet will still prompt for change comments; however, the comments will not be required.

This item meets the following DCAA requirements:
Any corrections to timekeeping records are documented, including authorizations and approval
Changes are initialed, authorized, and dated by the employee and supervisor, and include a description of the reason for the change.
Because Web TimeSheet is an automated timekeeping system, there are no actual initials/signatures. Initialization occurs when the approver/employee enters a change comment and saves/approves the timesheet or timecard entry. For timesheets, authorization occurs when the user is notified of the changes and takes appropriate action (either addresses the issue with the individual who made the change, or does nothing).

When you enable DCAA Compliance, the Allow blank resubmission comments permission is disabled for all users. If you later disable DCAA Compliance, this permission is not automatically re-enabled. Therefore, if you disable this feature and would like users to be allowed to leave timesheet resubmission comments blank, be sure to re-enable the permission for all users.