It could be the case that you have drawn a couple of pages of your comic and then decide that you have chosen for the wrong page format.
I know, the hair in your neck could stand right up at those moments as you see yourself doing all the work all over again. But there is no need for that as you can change the page format guide for all pages of your story in Manga Studio.
I hope this little video made the option (that you have probably have overseen) clear. So grab a coffee and get creative.
Have a great one.