I think that I know what you are trying to achieve. You want to make all posts at the beginning of the semester to hidden forums, as a way to prepare in advance. But the problem with this approach, as you have discovered, is that these original posts contain the date that the posts were made and not the date that the forum was unhidden.
No, as far as I know from using forums for 10 years, Moodle doesn't allow you to change dates on forum posts. Even editing a forum post (as manager) does not change its date. So, can't do.
Another problem with this "prepare" method is that I am not sure how Moodle handles the emailing of these posts. If a forum is hidden, I don't think that the post goes out. But I also don't think the post goes out when the forum is unhidden. So this situation needs careful thinking.
Also, I don't think one can create a new copy of a course from a backup and have all initial forum posts retained. (In Blackboard 9, one could do it. But Blackboard's forum tool was a real wimp compared to Moodle's.)
In my own practice, the only solution that I have found is that when I am turning a topic on for students, this is when I copy and paste initial forum posts from older courses. It just a step in my entire process, and even if I happen to have 8-10 topics, I can knock this off in several minutes. (Having Moodle open in side-by-side browser windows helps.) Yes, I wish that I could drag and drop old forum posts into a new course's forums, but this feature does not exist (yet.)