Overriding a variable in two bash scripts
===

I have to bash scripts: 

script1.sh

    HELLO=hello
    export HELLO
    ./script2.sh
    echo $HELLO

script2.sh

    echo $HELLO
    HELLO=world
    export $HELLO

The output is `hello hello` instead of `hello world`. How can I modify variables between scripts which call each other?

EDIT: Passing variables as arguments won't work. I don't know the number of variables which might be changed in script2.sh.