Single-spaced comments in changes package with double-spaced document

Quite likely you want all of your margin notes to be single spaced.

\documentclass{article}
\usepackage{changes,etoolbox}

\usepackage{lipsum}

\makeatletter
\preto\@marginparreset{\linespread{1}}
\makeatother

\linespread{2}

\begin{document}

\lipsum[1-1][1-3]\comment{I would like this comment to be single-spaced.}
\lipsum[1-1][4-6]

\end{document}

Explanation: \@marginparreset is the hook that the LaTeX kernel provides for doing initializations before starting to typeset a margin paragraph (internally used by the changes package).

Using this answer I tried

\documentclass{article}
\usepackage{changes}
\usepackage{lipsum}
\linespread{2}

\begin{document}
\lipsum[1-1][1-3]\renewcommand*{\baselinestretch}{1}\comment{\selectfont  
I would like this comment to be single-spaced.} 
\lipsum[1-1][4-6]

\end{document}

which seems to work. If you decide this is the way to go, then obviously one may make it more automatic and safer by patching this into \comment. You could just look up its definition from changes.sty and redefine it as follows:

\documentclass{article}
\usepackage{changes}
\usepackage{lipsum}
\linespread{2}
\makeatletter
\renewcommand*{\comment}[2][\@empty]{%
\edef\oldbaselinestretch{\baselinestretch}%
\typeout{\oldbaselinestretch}%
\renewcommand*{\baselinestretch}{1}%
\setkeys{Changes@comment}{#1}%
\Changes@output%
{comment}%
{\Changes@comment@id}%
{}%
{}%
{\selectfont #2}%
{\changescommentname}%
{\selectfont #2}%
\renewcommand*{\baselinestretch}{\oldbaselinestretch}%
}
\makeatother
\begin{document}
\lipsum[1-1][1-3]\comment{I would like this comment to be single-spaced.} 
\lipsum[1-1][4-6]

\selectfont
\lipsum[1-1]

\end{document}