Best Of

You have two options when converting projects to different aspect ratio panels: maintain or adjust to fit.
Stretching everything is OK when the aspects change is small, but pulling everything to twice its width creates terrible results as you have seen. Keeping aspect delivers pages and pops that use only part of the screen. But this is your best choice, as you can edit sizes and arrange positions to suit the new usable space to your benefit, without having to start over. It will still be a significant effort if you have lots of pages and pops.