Question
During a redesign of my site (which covers London theatre, so lists thousands of actors and shows) my developer changed the URL structure, so that the thousands of URLs in the Google index are now wrong. Obviously this is bad. It feels to me like there are too many to contemplate redirects for all of them. I assume it will take a lot of work to repair this - but where do I start? For now I've uploaded a new sitemap and attempted a kludge of using Webmaster tools that URLs using the old structure used parameters, so it should forget all but one of them - but I'm not sure if I'm missing another way to start fixing things.
Answer
There are never too many to redirect, no matter how many there are. 301 redirecting your old URLs is absolutely critical. Without those, no matter what you do, you will never reclaim any external link value pointed to those pages. It may not amount to much on a per-page basis, but added together it's probably pretty significant.
If the old URLs followed a consistent pattern and the new URLs do as well, a good programmer should be able to create global redirects on a site-wide level rather than having to do it on a page by page basis.
