After a long time of "testing", the button "old version" has finally disappeared from the new google analytics interface.
If you have been using the old version up to now, and for some reason you would like to revert to the old one, here is how:
Navigate to google analytics on a browser.
Create a cookie (or edit an existing) on your browser with the following data:
Expires Wed, 18 Apr 2017 12:25:02 GMT
Open a new tab and navigate to https://www.google.com/analytics/settings/home
Close the old tab with the new version :)
This should keep your default interface to the old version for as long as you don't navigate to the new version again (in which case the cookie will be removed by google), and for as long as the old version exists and checks for this cookie.
P.S. Dear google analytics re-designers and developers. Some of us spent days setting up dashboards to accommodate our needs in the old version of google analytics. Surely you can find a way to migrate our settings to the new version. Please do so.