Migrating _setDomainName from Classic to Universal Google Analytics - google-analytics

Question

I'm trying to figure out how to correctly migrate from Google's Classic Analytics to Universal Analytics, specifically regarding the _setDomainName property of Classic Analytics.
The site I'm doing this for has several sub-domains, and the current format of the Classic tracking code is what I've been using for years on all sub-domains of this site, as well as in the main (www) URL:
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'TRACKING ID']);
_gaq.push(['_setDomainName', 'sitename.com']);
_gaq.push(['_trackPageview']);
(function () {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = 'https://ssl.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
</script>
The Universal tracking code has a default format of:
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','//www.google-analytics.com/analytics.js','ga');
ga('create', 'TRACKING ID', 'auto');
ga('send', 'pageview');
</script>
I was reading somewhere that to handle the migration of the _setDomainName property I should replace the the following line in the Universal code:
ga('create', 'TRACKING ID', 'auto');
with
ga('create', 'TRACKING ID', 'auto', {'legacyCookieDomain': 'sitename.com'});
However, I came across something else that said to replace:
ga('create', 'TRACKING ID', 'auto');
with
ga('create', 'TRACKING ID', {
cookieDomain: 'sitename.com',
legacyCookieDomain: 'sitename.com'
});
I've seen other people saying that the Universal code doesn't require any code modifications to handle this.
Does anyone know for sure what I should do here? Thanks in advance for any help!

Answer 1

This
ga('create', 'TRACKING ID', 'auto', {'legacyCookieDomain': 'sitename.com'});
should work best for you if your intention is to continue tracking pre-migration visitors. It uses the "auto" cookie domain so that you can track across other subdomains, like whatever.sitename.com, and the "legacyCookieDomain" parameter which supports cookies that come from ga.js or urchin.js.
[Source: https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#legacyCookieDomain]
Setting the cookieDomain paremeter with a value of "sitename.com" is the same thing as setting the whole parameter to "auto".