[ruslan_mart]

.myElement.absolute {
      position: absolute;
      top: 500px;
}

(function() {
	var elem = document.querySelector('.myElement'),
		elemIsAbsolute = false,
		elemMaxTop = 500;
	
	window.addEventListener('scroll', function() {
		if(this.pageYOffset >= elemMaxTop != elemIsAbsolute) {
			elemIsAbsolute = !elemIsAbsolute;
			elem.classList[elemIsAbsolute ? 'add' : 'remove']('absolute');
		}
	});
})();