! create an associative list that maps { { c1 -> c2 } { c2 -> c1 } }
: make-swap-assoc ( c1 c2 -- alist )
    2dup swap [ 2array ] 2bi@ 2array ;

: swap-string ( s c1 c2 -- s' )
    [ make-swap-assoc ?at drop ] 2curry map ;


! Alternatively, use locals and write the following word and then map it over a string. This seems a bit clearer.

:: swap-char ( c c1 c2 -- c' )
    {
        { [ c c1 = ] [ c2 ] }
        { [ c c2 = ] [ c1 ] }
        [ c ]
    } cond ;


: swap-string ( s c1 c2 -- s' )
    [ swap-char ] 2curry map ;