How to automatically replace one letter with another before rendering

8-bit TeX engines, \usepackage[utf8]{inputenc}

% Redefine Yo/ye for umluatless version
\usepackage{newunicodechar}
\newunicodechar{Ё}{Е}
\newunicodechar{ё}{е}

Unicode TeX engines (LuaTeX, XeTeX)

Characters can be made active:

% Umlautless version
\catcode`\Ё=\active
\defЁ{E}
\catcode`\ё=\active
\defё{e}