This case cannot be reproduced.
Group assignments can be set as non-group assignments after students have submitted.
The group setting should be immediately locked after students have submitted.
Prerequisites: A course with at least 2 students
Create a group set and add the student to a group
Create a group assignment assigned to the group set
As a student, submit to the group assignment
Navigate to the assignment edit page, see that the option to modify the group assignment setting are able to be edited
Bonus round: Re-save the assignment and see the setting is now locked