\documentclass{article}
\begin{document}
\def\lettera{a}
\def\letterA{A}
\uccode`\(=`\[\lccode`\[=`\(
\uccode`\)=`\]\lccode`\]=`\)
\begin{tabular}{ll}
        & ABCDEFGHIJKLMNOPQRSTUVWXYZ \\
\TeX    & \lowercase{ABCDEFGHIJKLMNOPQRSTUVWXYZ} \\
\LaTeX  & \MakeLowercase{ABCDEFGHIJKLMNOPQRSTUVWXYZ} \\\hline
        & abcdefghijklmnopqrstuvwxyz \\
\TeX    & \uppercase{abcdefghijklmnopqrstuvwxyz} \\
\LaTeX  & \MakeUppercase{abcdefghijklmnopqrstuvwxyz} \\\hline
        & \i\j\ae\oe\lettera \\
\TeX    & \uppercase{\i\j\ae\oe\lettera} \\
\LaTeX  & \MakeUppercase{\i\j\ae\oe\lettera} \\\hline
        & \AE\OE\letterA \\
\TeX    & \lowercase{\AE\OE\letterA} \\
\LaTeX  & \MakeLowercase{\AE\OE\letterA} \\\hline
        & [FOO] \\
\TeX    & \lowercase{[FOO]} \\
\LaTeX  & \MakeLowercase{[FOO]} \\\hline
        & (foo) \\
\TeX    & \uppercase{(foo)} \\
\LaTeX  & \MakeUppercase{(foo)} \\
\end{tabular}
\end{document}
