In an existing Forum module, when I go to User Settings or Forum Administration, the display does not look right.  I can't click on anything to change my profile or settings.  However, if I add a new forum module to a sample page, it looks OK.

The existing forum module was created when we were using DNN 4.9.0.  I don't know what version the Forum module was at that time.  We have since upgraded to DNN 5.1.4 and Forum module 4.5.3.  I did not notice the User Settings screen being different because I hadn't been in that screen for a while.

I've checked the settings of the existing module and the new module I added.  They are using the same container.  I tried setting the container to one of the standard DNN containers, but when I go to the User Settings screen, it goes back to the site default container.  But the fact that a new forum module using the same container looks ok, leads me to believe it's not a problem with the container.

I don't want to have to re-do the entire forum module that I spent many hours to set up.  I don't want to lose all the posts in that existing module either.

Here is a screen print of the problem.

Your problems are all correctable in your custom theme for the module (or it was an older version with a different theme). When you put a new instance of the module on the page, it uses the updated default theme.