This JavaScript will automatically focus another control depending on the length of the current control's content. This should be called in a control's onkeyup event.

move-control.js

function move(current, next, previous, length) {
    if (next != null && current.value.length >= length) {
        document.getElementById(next).focus();
    }
    else if (previous != null && current.value.length <= 0) {
        document.getElementById(previous).focus();
    }
}