function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading) {
        return;
    }
    if(newValue == '') {
        if (g_form.getValue('category') == 'Hardware' && g_form.getValue('u_incident_type') == 'Dental') {
            g_form.clearOptions('subcategory');
            g_form.addOption('subcategory', 'Computer', 'Computer');
            g_form.addOption('subcategory', 'Document Scanner', 'Document Scanner');
            g_form.addOption('subcategory', 'Intraoral Scanner', 'Intraoral Scanner');
            g_form.addOption('subcategory', 'Topaz Signature Pad', 'Topaz Signature Pad');
            g_form.addOption('subcategory', 'X-Ray', 'X-Ray');
            g_form.addOption('subcategory', '3D Scanner', '3D Scanner');
        }
    }
}

This is working in the backend\instance version of the Incident form. Glad I've made some progress cause this was driving me nuts.