While the Discussion Board Forum topic itself did migrate into Canvas, any instructor-made threads within the Discussion Forums do not come over.
Here is a picture of the Forum title and Description that is migrated into the Discussions area in Canvas:
And here is a picture of the threads and post inside a forum, these do not get migrated into Canvas:
You will need to re-post any threads in the Canvas course offering moving forward if you want that functionality:
You can also create the threads as main Discussion topics in Canvas so that they do carry over to your future course offerings: