It depends.
Like you, yes I modify challenge layouts. For example, the 20 things layout for the party - I submitted, then went back and changed a few things plus added journaling to go with the story. That I do all the time, especially with challenge layouts.
Paper/old printed photos: Sometimes. Most often, I'll use the same photo to tell a different story, or flesh out a story more like Brendazzle. I only have a few photos of me growing up on the farm, but I have more stories to add. So, I let the star shaped photos lie on the previous page, and use them again, non star, to tell a new story.
That said, there are a few pages that I originally paper scrapped and am redoing digi. My wedding album I started in paper, and then did some digi. I'm going to redo the few paper layouts, so the book flows better.
But, one of my yearly albums is half paper, half digi. So, I scanned the paper, fixed the journaling (blue ink and purple paper? Ugh - can't read it!), and the paper page will get reprinted in with that year when I finish it digitally. I'm not redoing it, just adding it to the book it belongs in.
