\unexpanded\def\uppercasing{\addff{uppercasing}}
\unexpanded\def\lowercasing{\addff{lowercasing}}
\definealternativestyle[UpperCase][{\bf\uppercasing}][]
\definefirstline [UpperCaseWords] [
alternative=UpperCase,
n=1,
]
\starttext
\setfirstline[UpperCaseWords] \input knuth
\stoptext
</context>