Thank you for writing in, did you inherit that website? 3.x to 6.x is a long jump and yes it will break things and it will need adjustments.
I see that pages are built-in Visual Composer, I would advise that you rebuild those pages using Cornerstone. Rebuilding those pages should not be that time consuming since you already have the design and content, that would actually save you time in the future because of continuity (easier to maintain because you're the one who created it).
You can rebuild the pages first on a draft page then save that page as a content template so you can load that to your live pages later.
In Cornerstone, you can make a Section a Fullwidth by turning OFF the Row's Inner Container option, leave it On if you don't want the content of the section to go the edge of the page.
To make sure that your page can go Fullwidth, please use a No Container, Header | Footer page template.