When people first start up a WordPress blog they usually go for the default settings as this is by far the easier option. Unfortunately in certain cases it is the worst thing that you can do. This is especially so when configuring your permalink settings. I had completely forgotten about this until I read some of the comments on Understanding The New Rules Of SEO.
The problem arises in the fact that the default setting is http://wassupblog.com/?p=123 which shows the search engines that you have posted post # whatever but not much else. It makes more SEO sense to present to the search engines a lot more information such as the title of the post. While you can change the value midstream this can have disastrous results because all your pages that were previously indexed by the search engines become invalid. Not only that, if you are in the habit of linking to pages within your own blog, a good habit I might add, these also become invalid which would mean you going through all your posts and fixing up the links manually. A very boring and time consuming task, as can be attested by some of the commentators on the above post that I mentioned.
There is an easier solution, a plugin by Dean Lee called Permalinks Migration Plugin. As the author states;
There is a way to tell the search engines (and browsers) that the page has permanently moved, and that the old address should be replaced by the new one . It’s called a “301 Redirect”, also known as a Permanent Redirect.When you do this, Search engines will update their indexes quickly and you won’t lose your pagerank.you will continue to receive traffic as though nothing had changed. This works for search engines, bookmarks, and links from other sites.
So, if you are having permalink problems or changing your permalink structure I think you will find this plugin very useful.