How can I change case in text not defined by text format field?

Install WordService from the App Store.

Services are extra little features added to your Mac, which appear via the Services menu option under the App menu for every App they are useful.

So with your text selected and WordService installed:

Menu > Pages > Services > WordService: Initial Cap of Sentence/Initial Cap of Word/lower case

Important: you will only see a Service in the menu if you have something selected that it can do. You can turn them off or on in:

System Preferences > Keyboard > Shortcuts > Services

Most of the WordService services require that you enable them before they work.

System Preferences : Keyboard panel : Shortcuts : Services, and then scroll downward until you encounter the following (an abbreviated view):

Check the ones that you need and they will then be available from the Pages : Services menu when you have text selected for them to act on. Hover your mouse over these names that are too long (...), and the full name will be revealed.

To see a list and description of all individual services, there is a WordService application installed in your /Applications folder (and available in Launchpad) with the following icon:

When you run it, click the Help button and a PDF document will open in Preview with the Service descriptions. That is the sole functionality of this application.