How do you select all to change formatting for the entire book, all the chapters as one?
Once you have all of the files selected that you want to convert, use the Documents/Convert/Formatting to Default Text Style menu command. As it sounds, this will convert documents to the default, which is set up in the Editor options tab. There isn’t a tool for changing bulk text to some arbitrary formatting at the moment.
As with all large-scale changes, I recommend setting aside a backup of the project prior to running it, just in case a mess is made.