Hi Adam:
You can select all of the text across multiple pages by clicking in the text with the Type tool, and choosing Edit > Select all... if the text is in a series of threaded text frames.
If you have designed the document with multiple threaded frames (or worse, disconnected frames) you will need to repeat this multiple times... unless you are using paragraph styles.
If you are using paragraph styles, you can just edit the style definition(s). When using paragraph styles, be sure to create a base style, so that changing its definition will roll though the other styles.

As someone who specializes in long document layout, I learned the hard way to design my files to be ready for all those last minute formatting adjustments that almost always start with, "Hey Barb, did I tell you you need to update X?" And when they phrase it that way, we both know haven't mentioned it yet.
Primary frames are worth adding to the list because page size and margin changes come up just as often.
~Barb