Currently changing the group mode in a forum does not update any previously existing forum discussions to use groups. This can lead to discussion threads that are erroneously shared outside of the desired groups with no way to fix short of manually updating the groupid column in the forum_discussions table in the database. It would make sense for discussions with no group to dynamically update to respect the current group mode setting of the forum activity.

Steps to reproduce

In a course, enroll at least two students, A and B
Put the students into 2 different groups, A and B
Create a forum activity with no groups mode set
Logged in as student A, make post 1
Logged in as student B, verify you can see post 1
Edit the forum activity and change the groups mode to separate groups
Logged in as student A, make post 2

What happens now

post 1 still has no group assigned, and Student B can still see post 1
post 2  has a group assigned, and Student B cannot see it

Possible improvement

Post 1 and Post 2 both have a group assigned, Student B cannot see any posts