A primary field is defined by its behavior under a conformal transformation $x\rightarrow x'(x)$:
$$\phi(x)\rightarrow\phi'(x')=\left|\frac{\partial x'}{\partial x}\right|^{-h}\phi(x)$$

It's fairly easy to see that the gradient of the field doesn't have this nice property under the same transformation, it gets a non homogeneous term. Still, is it possible to construct a derivative that would behave nicely under conformal mappings and give the usual derivative for Lorentz transformations? By adding a "connection" similarly as what is done in general relativity or gauge theories. And if not, why?


Question: What is the proper way to change the section title font in LaTeX?
Answer: I would say that really much depends on what you are trying to
achieve. 

Do you want to make a class that others can use as well? You can
define your own commands for sectioning. Are there just minor
changes within one of your documents? Use package etoolbox and
patch your changes. Are there bigger changes to the sectioning
and titles? Package titlesec is commonly used with the standard
classes.  

KOMA-classes have their own way of defining the font for various
instances of a document. The following example shows the
interface. 


  Disclaimer: This is just for demonstration, nobody should use
  that many stuff in just one document.




\documentclass[plainheadsepline,headsepline=2pt]{scrreprt}
\usepackage{blindtext}
\usepackage{scrlayer-scrpage}
\usepackage{xcolor}
\RedeclareSectionCommand[font={\Huge\fontfamily{qhv}}]{chapter}
\RedeclareSectionCommand[font={\rmfamily\footnotesize\itshape}]{section}
\addtokomafont{captionlabel}{\usefont{T1}{qzc}{m}{it}}
\addtokomafont{caption}{\color{red!80!black}}
\addtokomafont{headsepline}{\color{red!80!black}}
\begin{document}
\chapter{lazy Leguan}
\section{walzing wombat}
\captionof{figure}{A figure caption}
\blindtext
\end{document}