LuckyLiz - 2:37 pm on Jun 10, 2013 (gmt 0)
My site isn't as big as those mentioned, but is one of the top sites in its niche - at least among the independently owned sites. We have done several site updates and redesigns over the years, the last one being to move everything into a CMS that uses URL rewriting to match our file names and have new material have the same file extensions as the old. We did see some loss from the change to the CMS, but it was minor and appeared to be a one-time hit and I suspect because at the same time we had to 301 redirect a number of important pages.
We added a mobile version of the site a month or two before traffic started to slide. That does not appear to get enough traffic to account for the huge drop in traffic the desktop site has seen.