A few weeks ago I posted about some small issues that I see over and over again in SharePoint 2010 migrations - you can read about them here:  Quick Tip - SharePoint 2010 Migration - Legacy Views and Add Icon

Discussions come with their own set of headaches requiring resolution after a migration.  The symptom is usually the same: clicking on an item in the discussion board web part does not take you to the full screen flat (or threaded) view of the topic. 

Resolution

There are 3 potential resolutions to this issue:

1.      The web part needs an update

Sometimes the web part just needs a simple refresh to kick itself back into gear.  To do this:

1. Edit the Discussion Board Web Part on the landing page
2. Ensure the view is set to "Subject with Replies" (or just "Subject" for older sites)
3. Click OK
4. Exit Edit Mode

To ensure it is working as intended, click on a a topic in the web part; you should be directed to a new page with a more enhanced view of the discussion

2.      The "Subject" view on the back end need an update

When the above steps do not resolve the issue, that may indicate that the problem lies with the views within the list - in this case the problem child is the "Subject" or "Subject with Replies" view which needs to be recreated.

1. Click on "Team Discussion" from the landing page or the left nav
2. Go to the discussion board à List tab on ribbon
3. Create View
   1. Choose "Subject View" in the view format section
   2. Call it Subject2
      1. Make it the default
      2. Keep all other settings the same
4. Modify the original view ("Subject")
   1. Delete it
5. Modify the new view ("Subject2")
   1. Rename it "Subject"
   2. Change the URL to "AllItems".aspx
6. Update the web part on the home page to use the "Subject" view (follow steps in the previous section)

3.     The "Flat" view on the back end need an update

If none of those solve the problem, the issue may be with the "Flat" or "Threaded" view.  In this case, the Flat or Threaded view has a bad setting for the "Show In" property of the view.  You can check for this:

1. Click on "Team Discussion" from the landing page or the left nav
2. Go to the discussion board à List tab on ribbon
3. Select "List Settings"
4. Navigate to the bottom of the list settings page and review the views - you should have:
   • Threaded --> Shown in Discussion
   • Flat --> Shown in Discussion --> Default
   • Subject --> Shown in Top Level --> Default 
5. ...in views that are corrupted, the Flat or Threaded views will have a Show In property of "All"

To correct this issue (using the "Flat" view):

1. Click on "Team Discussion" from the landing page or the left nav
2. Go to the discussion board à List tab on ribbon
3. Create View
   1. Choose "Flat  View" in the view format section
   2. Call it Flat2
      1. Keep all other settings the same
4. Modify the original view ("Flat")
   1. Delete it
5. Modify the new view ("Flat2")
   1. Rename it "Flat"
   2. Change the URL to "Flat".aspx
   3. Set it to be the default view
6. You may still need to update the web part on the page to "refresh" it as described in the previous section