\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\DeclareTextCommandDefault{\blub}{blub default!}
\DeclareTextCommand{\blub}{T1}{blub in T1!}
%\UndeclareTextCommand{\blub}{T1}
\begin{document}
\blub
{\fontencoding{OT1}\selectfont \blub}
\end{document}