recode <- function(x, old, new) {
    u <- rep(NA, length(x))
    for (i in 1:length(old)) {
        if(is.na(old[i])) {
            u[is.na(x)] <- new[i]
        } else {
        u[x == old[i]] <- new[i]
        }
    }
    return(ifelse(x %in% old, u, x))
}

a <- sample(c("A", "B", "C", "D", NA), 20, replace = T)
recode(a, old = c("A", NA), new = c("E", "F"))