Having worked on Advanced forums for the last 5 years, ever since HSU first created it as a patch to core forums, I can say it would not be like the question engine upgrade in 2.1.
It is a patch to core that adds additional functionality. There would be no conversion process it would just be adding functionality to existing code. The only reason it is a separate module at the moment is because we were unsure how core wanted to proceed with forums and our clients demanded the extra functionality ASAP. So to be able to maintain it long term and allow clients to also use Moodle forums that would upgrade with Moodle core's decisions we made a separate activity.
We have kept Advanced forums up to date with all changes to Moodle core forums as well. It would be a drop in solution not a conversion or migration. In the code for Advanced forums you should be able to replace hsuforum with forum and slap it on top of forum and it will work.