@DWorks your original post says you have a free account - if this is correct, the only way to rebuild is to create new pages using the child theme for each page (one at a time), update the current/live URL to a temp URL, add the previous URL to your new child theme page, publish the new page, and unpublish the old page.
If you have Content or Marketing Hub Pro, you have access to Content Staging which makes the whole thing a lot easier. I can share details if this applies to you.
There is another way, but you have to be careful and understand the pitfalls.
In the page settings, you can change the theme from the marketplace theme to the child theme. When you do this, the template should update to your base.html from the child theme and you should see your footer changes live.
Caution - the module IDs will still be tied to the marketplace theme, not the child theme. What does this mean? If you clone a module from the marketplace to your child theme and modify it, those changes wouldn’t be reflected on any pages you already built with that module from the marketplace.
To avoid confusion in the future with updates made to the child theme, once you change the template for each page from the marketplace theme to the child theme you could drag a new version of the module onto the page and rebuild it.
The modules in the sidebar will be from the child theme (even if you haven’t cloned any, again the ID will be different in the background). Once you rebuild your content you can delete the original module and move to the next, and so on.
You shouldn’t have to replace the header/footer on any page UNLESS you made changes to the header/footer files in the child theme (not really recommended).
Happy to talk through specific questions about getting everything converted if this is unclear.
Another option would be to reach out to SpaceRockX, they wrote a script that will swap the marketplace ID with the child ID behind the scenes so you don’t have to manually udpate everything. I’m not sure if there are limitations based on your subscription to do this, but they can help answer your questions if there are.