Hi Again,

The (Not provided) category isn't showing up in March 2011 because they only started obscuring data from logged in users in October of 2011. This might account for at least some of the traffic difference between 2011 and 2012.

I think this is the blog post that they announced the change via:

http://analytics.blogspot.com/2011/10/making-search-more-secure-accessing.html

You can potentially get some idea of what terms the (not provided) traffic is coming in on by looking at the landing page. You can also use PPC data, but I'm a little fuzzier on how that is done as AdWords isn't something I use a whole lot of.

Have you looked into long-tail traffic yet, or the traffic being generated by (not provided) search terms?

I've found word clouds to be useful at analyzing changes in long-tail traffic. If you can pull April and  March search terms into two different spreadsheets, then use something like Wordle you can get a really generalized overview of your long-tail makeup.

Similar to RankSurge's answer - The other thing you might want to consider is if the format of the SERPs has changed. Are there video, photo, local, or other universal results-or rich snippets-that didn't used to show up alongside your site?

To me, that's not what your 404 page is for. I wouldn't recommend doing it. What happens if they typo something, or hit a bad link some other way? There's no message to tell them what happened and it creates a confusing user experience.

Better, in my opinion, to redirect the file individually

Arti, there's a good article on SEOmoz about redirects. Down the bottom, there are some how-to guides on how to do things like 301's. Just be careful of infinite loops if you're using apache.

http://www.seomoz.org/blog/url-rewrites-and-301-redirects-how-does-it-all-work