Well, I got this old blog and because it's old, the permalink structure is not really good. Then I updated it recently but this brings me to another question. How do I update my sitemap to keep the damage as low as possible? You see, all urls in my sitemap and also in GG results were blog/post?id=1 etc. Now the urls are blog/key-phrase and so on. Can I just rewrite the sitemap with the new urls and let GG deal with it the way she wants? Thanks.
You need to make sure you permanently redirect the old URLs to the new URLs if you want to maintain their PageRank from outside backlinks.