The best time to load a lot of new content is before the course begins and your entire course is unpublished. However, it looks like that ship has sailed.
If you are importing or copying items from a previous course, then if the item is published in the original course, it will transfer over as published. If the item is unpublished in the original course, it will transfer over as unpublished. (e.g. published >>> published; unpublished >>> unpublished). If you are creating new content in the target course, it will be unpublished.
The problem you have is that the content from the original course is all published, and if there are student submissions, you can't change that. Here's the solution: Set up everything in a sandbox course. You can import everything into the sandbox course, and unpublish all of the items. Then import (or copy) the unpublished items into the target course.
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