Make it a template with a dummy parameter that defaults to T - this is to postpone type deduction to the point where the function gets instantiated, otherwise SFINAE doesn't work. Then you do the check you want in default value of another parameter.
template<
typename U = T,
typename = typename std::enable_if< std::is_same<U,const char>::value >::type
>
operator Range<const unsigned char>() {
return *reinterpret_cast<Range<const unsigned char>*>(this);
}