Test code without no_par

% Code based on <https://tex.stackexchange.com/a/491861/73317>:

\documentclass{article}
%\usepackage[T1]{fontenc}
\usepackage{xparse}

\ExplSyntaxOn

\seq_new:N \l_evan_letters_seq

\cs_new_protected:Nn \evan_make_upper:n
  {
    \str_upper_case:n {#1}
    \cs_set_eq:NN \evan_capitalise:n \evan_make_lower:n
  }

\cs_new_protected:Nn \evan_make_lower:n
  {
    \str_lower_case:n {#1}
    \cs_set_eq:NN \evan_capitalise:n \evan_make_upper:n
  }

\cs_set_eq:NN \evan_capitalise:n \evan_make_upper:n

\NewDocumentCommand \toggle { +m }
  {
    \evan_capitalise:n {#1}
  }

\ExplSyntaxOff

\begin{document}

  \toggle{Something like this\par with paragraphs}
  \toggle{or this\par too}
  \toggle{or this!}

\end{document}