Preview first. Then you can opt in to stay if you wish and/or and provide feedback.
They probably would have gotten more press but just turning it on, but, no doubt after seeing reactions to changes in Facebook and more recently Google News, the majority might have been negative.
People by nature are resistant to change and Flickr still is one of the largest repositories of photos on the Internet.
Apparently the gang at parent company Yahoo! weren't taking chances as Facebook (in a much different way) is catching up quickly with photo hosting and Google's Picasa client software continues to improve somewhat dramatically ...... Not to mention the fact that Google recently purchased Picnik, the start-up used by Flickr for online editing.
After logging in, you'll probably be greeting with this banner:
1) The default photo size is now bigger.
2) They've (finally) increased the overall page size.
3) An new 'actions' menu consolidates things.
4) More privacy options (Did we see this coming? :)
....and a variety of other changes and new features.
Most users can now opt in to the 'new' Flickr here.
Update: According to this post by Josh Lowensohn at CNET, the opt-in period will last one month at which point all users will see the 'new' Flickr.