Hi, I imported 31 forums and now I am trying to make a few to Galleries.

However I looked how to and found in
Control Panel » Forum Settings » Edit Forum
That it is not possible to enable the Gallery function.

Do I really have to create new forums and move topics and then delete the old forums???

Is Gallery Forum? No
You may not toggle the gallery status of a forum that has already been created.

No one here with answer???

you can not take gallery posts and move them to a regular forum or from a regular forum to a gallery nor can you change from a standard forum to a gallery forum or from a gallery forum to a regular forum.

The gallery forum uses a different format and structure.

Your cut and paste in red from the control panel message should be self explanatory thus an answer really is not required since you gave the proper answer with your question.