I want to switch to a different font in Writer documents without changing the default font.
Yes, you can. The default fonts are set in Tools > Options > LibreOffice Writer > Basic fonts (Western), you don’t need to touch these unless you want to change the default fonts for all new documents.
In a document, to change to a different font select some text and click a different font in the Font Name box in the Formatting toolbar, or in the Sidebar under Properties > Character. This is called direct formatting and is a quick and dirty way to change fonts in a short document. If there are going to be further revisions, the document will be added to, you share it with others, it is part of a larger project, etc. this will make more work in the long run.
You can test that you haven’t changed the default font by selecting the text in which you just changed the font and clicking Format > Clear direct formatting. The text will revert to the default font; in fact to the style of the paragraph, which is probably Default paragraph style and to the style of the character which is Default character style.
LibreOffice is more sophisticated in its use of styles than another word processing program so there are styles for character, paragraph, lists, frames, tables, and pages. By changing the attribute of a style you can change that attribute in every instance in the document (unless the text has been direct formatted).
Paragraph styles are the place to start. When you begin writing the body of a new document the style you should be using is called Text Body (not Default paragraph style, despite its name). Click in the text, then in the Sidebar double-click Text Body, your text will take up that style. You might not see much difference except line and paragraph spacing, but that even change should make it easier to read.
You can change the default for this document only by modifying the style. Right-click on Text Body in the sidebar and select Modify, in the dialog that opens select the font tab. Liberation Serif is currently selected, change it to Liberation Sans and click OK, your text will immediately change to the new font, you can change it back if you want. The Headings listed in the sidebar are better to use than direct formatting as they will appear automatically in a table of contents.
Plenty of information in the Writer guide available for free download on the . Cheers, Al
In addition:
[Professional text composition with Writer](https://wiki.documentfoundation.org/Videos/Professional_work_in_Writer)