First, it's not clear whether you can do this in any document, or whether you have to update the template on which all the documents have been based. I assume that will be clarified in the final manual/help files, but I've found that some things work if you update the template, but not all.
In the "Normal" template that I have based all the documents in this project (which began as 1.97 project) I have the main heading for each document formatted as heading "Heading 1".
I decided to change the font for all Heading 1 paragraphs, So, I chose the new font, opened the Styles Panel, right-clicked on "Heading 1" and selected the option "Redefine paragraph style from selection", with the options "save all formatting" and "include font family" selected. Small thing, but doing do that causes the paragraph that was tagged as Heading 1 to lose all formatting, so you have to re-format it with the new style, (No big deal, obviously, but I assume it's not mean to work like that).
I click on it again and apply the formatting (and/or use the keyboard shortcut - same result), but the format of existing paragraphs that use that has not been updated, and if I click on them and apply the style again, they still don't update. But if I type new paragraphs and apply the style, they update properly.
Might just be an artifact of importing an old project into v3, but I hope it's useful to report it.