JSFiddle - React, Tailwind, and code Playground

HTML

<form action="post" method="">
    <label for="pick_org">1. Pick Organization</label>    
    <select name="pick_org" id="pick_org">
        <option value="1">Org 1</option>
        <option value="2">Org 2</option>
        <option value="3">Org 3</option>
        <option value="4">Org 4</option>
        <option value="5">Org 5</option>
    </select>
    
    <label for="pick_event">2. Select Event</label>
    <select name="pick_event" id="pick_event">
        <option value="A" class="org-3">Event A</option>
        <option value="B" class="org-3">Event B</option>
        <option value="C" class="org-5">Event C</option>
        <option value="D" class="org-1">Event D</option>
    </select>
</form>

JavaScript

$.fn.changeToHidden = function () {
    this.each(function(i, ele) {
        var $ele = $(ele),
            $new = $('<input type="hidden">').attr('rel', $ele.html())
                                             .attr('value', $ele.attr('value'))
                                             .attr('class', $ele.attr('class'));
        $ele.parent().append($new);
        $ele.remove();
    });
};

$.fn.changeToOption = function () {
    this.each(function(i, ele) {
        var $ele = $(ele),
            $new = $('<option>').html($ele.attr('rel'))
                                .attr('value', $ele.attr('value'))
                                .attr('class', $ele.attr('class'));
        $ele.parent().append($new);
        $ele.remove();
    });
};

$("#pick_org").change(function() {
    var org = $(this).val();

    $("#pick_event option").changeToHidden();
    $('#pick_event input[type="hidden"].org-' + org).changeToOption();
});