Rog,
Depends on what you call tweaking! If you change the format or size,then yes, you need to create a new book, I tried and wanted to do my large Landscape, 11x13 and then 3 smaller formats and they said it can't be done! If on the other hand, you used the older version called BookSmart(?), there seems to be a button somewhere where you can change the format/size.
If just moving images, type,words, then you just change it and call it fixed or something and then upload it again. I JUST today got my tweaked 2nd edition delivered today! I added pages, made the font on ALL the stuff smaller and corrected a few spelling errors,
Rich