How to replace Titles styles without starting over?

I don't have the callout titles, but I tested with some of the built-in titles. Drag your new title over the one in the timeline and hover until the old title has a white bounding box, then release the mouse. Choose "Replace from start" and the new title should replace the old while keeping the text. I don't have the Callouts titles you're using so it's possible this won't work but it's easy enough to test.

You can select (command-click, or drag a selection) ALL titles that you want to change text attributes for and go into the Text Inspector and make global changes to those that are selected, including Styled text (from the top menu that usually displays "Normal").

You can also Save the attributes and formatting of text into a Style for future use (so you don't have to manually set multiple options.) Take it from me, you can spend a long time customizing the looks of text, especially with the 3D Text options and when you get something you really like, it is best to save it (format AND attributes).

HTH