Depending on what theme you are using, you might lose some text when switching sizes. If I ever want to do something like this, I duplicate my book first.
Duplicate your book:
You can do this by right clicking (or holding control while you click) on the name of the book on the sidebar on the left. You will get an option to duplicate.
Then either on the original or duplicate, click on Change Theme in the upper right hand corner and select the larger size.
I just tried this with one of my previously printed books, and it seemed to keep everything on the correct pages with the same text. I think because you're going to a larger size, you won't lose anything, but I always think it's a good idea to duplicate it first as a backup.
I hope this helps!