\startbuffer[demo]
<p>auflösen, fließend</p>
\stopbuffer

\startxmlsetups xml:initialize
         \xmlsetsetup{#1}{p}{xml:*}
    \stopxmlsetups

\xmlregistersetup{xml:initialize}

\startxmlsetups xml:p
    \xmlflush{#1}
\stopxmlsetups

\definefontfamily
    [mainface]
    [rm]
    [EB Garamond]

\setupbodyfont
    [mainface, 16pt]

\replaceword[sellig][auflösen][au{fl}ösen]
\setreplacements[sellig]

\starttext
     \xmlprocessbuffer{main}{demo}{}
\stoptext