I have seen this happen to a wide range of sites, some owned by me and managed by myself and some that are totally unrelated. There is only one pattern: That’s change and get dropped for three months. It’s totally absurd to penalize legitimate sites for updating their content, I mean what does Google want – a bunch of old content in it’s index?? (I know this has been the common moan for years)
Google really needs to tweak it’s algo so it realises that a site mostly the same has not been sold or switched for spam. How hard can it be? Example: Site A is still on topic and only 60% has changed, the likelihood is that it’s the same site, not some dodgy spam. These filters seriously need to be relaxed by a great deal in my opinion.
We all know that their are exceptions, if the site is trusted massively, then it’s probably not going to drop – but for most sites, they are not authority sites that have that trust box checked and thus are going to take a long absence from the SERPS.
Matt Cutts: I implore you to review this nano part of the Algo and let sites have more flexibility to update and keep themselves current.
I just updated and refined my sites homepage content. Wish i read his first.
Only 60%? 60% is quite a lot (depending on the size of the site).
Can we have more detail about what you change that causes these drops?
If you change the URL of a page or post for example then all the links that used to point to it from external sites will result in 404′s unless you 301 them – that page will probably lose all rank it used to have in the index.
I’m just saying that when you make big changes to a site you need to be very careful not to mess up your SEO. It may not always be the fault of Google’s algorithm.