If you are using doubleclick.js tracking code in classic snippet (ga.js or urchin.js) for remarketing, then migrating to universal anlytics can create problems.
Remarkeing through GA – At present, it is not possible to do remarketing with universal analytics because, it is not possible to embed doubleclick snippet in universal analytics snippet. Using Universal analytics, it is not possible to do remarketing through GA.
Demographics Report – It is not possible to enable demographic report in google anlytics account that uses universal analytics snippet.
If you are doing remarketing using GA & make use of demographic reports, then think twice before upgrading to universal analytics snippet from classic.Google+