I've been trying to track down some estimates on how much of a traffic decline I should expect to see in the short term after implementing URL rewriting. I would implement all the best practices for URL rewriting (301s on page by page basis, update all links to old pages, etc). Even anecdotal info about your experiences with this would be helpful to me.
I thought there would be more case studies out there, but I guess not.
I've been trying to track down some estimates on how much of a traffic decline I should expect to see in the short term after implementing URL rewriting.
That is very difficult to determine. There are a multitude of factors that will come into play.
Based on my experience, doing everything at once may not be the best option if you are reliant on that free traffic for income.
I might suggest doing it in pieces. Take your least trafficked area and start 301'ing those old URIs to the new URIs. Sit back for about 30-60 days and watch what happens. In most instances, you are going to see a dip during that period while the old and new get merged and purged. Once the new gets firmly seated, things should return to normal. But, there are factors here too that will have an impact. Getting inbound links updated would be to your benefit for long term success.
If you are pleased with the results of your first test set, I'd then move on to the next set to be redirected.
Doing them all at once will make the impact of shuffling between old and new much more noticeable and it will affect the bottom line.