With the default theme, you can just copy the default title page layout from beamerinnerthemedefault.sty
\defbeamertemplate*{title page}{default}[1][]
{
\vbox{}
\vfill
\begingroup
\centering
\begin{beamercolorbox}[sep=8pt,center,#1]{title}
\usebeamerfont{title}\inserttitle\par%
\ifx\insertsubtitle\@empty%
\else%
\vskip0.25em%
{\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle\par}%
\fi%
\end{beamercolorbox}%
\vskip1em\par
\begin{beamercolorbox}[sep=8pt,center,#1]{author}
\usebeamerfont{author}\insertauthor
\end{beamercolorbox}
\begin{beamercolorbox}[sep=8pt,center,#1]{institute}
\usebeamerfont{institute}\insertinstitute
\end{beamercolorbox}
\begin{beamercolorbox}[sep=8pt,center,#1]{date}
\usebeamerfont{date}\insertdate
\end{beamercolorbox}\vskip0.5em
{\usebeamercolor[fg]{titlegraphic}\inserttitlegraphic\par}
\endgroup
\vfill
}
And modify it, deleting the argument and swapping title and subtitle:
\documentclass{beamer}
\makeatletter
\setbeamertemplate{title page}
{
\vbox{}
\vfill
\begingroup
\centering
\begin{beamercolorbox}[sep=8pt,center]{title}
\usebeamerfont{title}\insertsubtitle\par% \inserttitle -> \insertsubtitle
\ifx\insertsubtitle\@empty%
\else%
\vskip0.25em%
{\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\inserttitle\par}% \insertsubtitle -> \inserttitle
\fi%
\end{beamercolorbox}%
\vskip1em\par
\begin{beamercolorbox}[sep=8pt,center]{author}
\usebeamerfont{author}\insertauthor
\end{beamercolorbox}
\begin{beamercolorbox}[sep=8pt,center]{institute}
\usebeamerfont{institute}\insertinstitute
\end{beamercolorbox}
\begin{beamercolorbox}[sep=8pt,center]{date}
\usebeamerfont{date}\insertdate
\end{beamercolorbox}\vskip0.5em
{\usebeamercolor[fg]{titlegraphic}\inserttitlegraphic\par}
\endgroup
\vfill
}
\makeatother
\title{My title}
\subtitle{sub}
\begin{document}
\maketitle
\end{document}

Of course, if you want to use another theme, you might need to modify the related title page layout.
edit: If you also want to swap the font, you are free to do this, just swap the arguments of \usebeamerfont as suggested by OP. I am not sure if this is always desired, that's why I rejected their edit. Here the according picture, if you do change the fonts.

title page. You can modify it in order to get what you want. One example of a modified template you can find here: https://tex.stackexchange.com/questions/116077/presentation-beamer-title-page – nox Sep 12 '18 at 11:50