Can I select all the text having the same style?
I want to change the style assigned to some of the text in one document. Style “B” should be applied to the text with style “A”. It would be very easy if I could select all the text in a document with style A, then apply style B.
The two current answers both focus on paragraph styles (the only kind of “styles” search available from CTRL-H, I believe).
If you want to select character styles (e.g., change all bold to underline for example), then you probably need the the Extended Search/Replace extension. There’s a step-by-step guide in another Q&A to using it to replace paragraph styles (and see this related one, too), but it can also replace character styles (you might want/need to set up some new custom ones).
It’s certainly worth experimenting with, as it is a very powerful tool for manipulating the text.
@Pipe
Find and Replace provides the ability to change one Style to another as an extended option that is normally hidden.
Edit > Find and Replace > More Options > Search for Styles. Then you have a choice of Find and Replace one by one, or Replace All.
Have you took look at Find and Replace dialog (Ctrl+H) options? See “Search for Paragraph styles” under Other Options.
From Libreoffice Help: Finding and replacing in Writer, paragraph “Finding styles”
"You want to find all text in your document to which a certain Paragraph Style is assigned, for example the “Heading 2” style.
In my opinion this is the correct answer for the question asked in the heading. See also this question where this strategy is used to count words.