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Make sure your new pages are optimized as well or better than the previous versions. Make sure they target the same keywords. Make sure to setup 301 redirects for any changed URLs. Etc, etc.
I've seen absolutely no evidence that might suggest a drop in traffic as a result... just make sure you keep everything technically correct.
I am targetting somewhat different keywords on my pages. That's what I meant by substantial changes and I should have said that. Should that take a little while to kick in?
EDIT: To clarify, I've changed things like h1, meta description, title, and link graph.
[edited by: Tonearm at 2:33 pm (utc) on May 3, 2009]
So if you're already using 301 redirects, it's a good idea to make sure they go to the new url in just one hop. And it's a VERY good idea, if you have a larger database driven site, to use a test environment and really "kick the tires". try all kinds of scenarios and monitor the http headers.
Another place where sites get tripped up is introducing new canoncial errors - especially in their url rewriting. There's a shortcut that some call "keyword fluffing". With this shortcut the database keys off a number in the url, and then there's also a keyword in the filepath. But that keyword can be ANYTHING AT ALL as far as the data query is concerned. It was just stuck in there to get a "keyword in the url" factor working.
This is NOT a good idea, and it opens up complex canonical problems that can delay or permanently diminish link the juice flowing through 301 redirects. As Asia_Expat said above, "make sure you keep everything technically correct."
Use Site Search [webmasterworld.com] before beginning a new thread - most situations are already quite well covered in previous discussions.