You can use the following trick to move an existing Comments box to a different page:
1. Open the Wix editor, and click on the Comments box that you want to move
2. Enable the "Show on all pages" option in the Editor Toolbox on the right:
3. In the Wix editor, go to the target page (the page that you want to move the comment box to).
4. The comment box should appear there, click it once and now disable the "Show on all pages" option.
Congratulations, you moved your comment box to a different page. Now you can move it around, resize it, etc. to fit the new page.