Many times I've seen a web site undergo a revision and everything is brand new, even brand new links!
You've worked so hard to get those great page ranks for all your pages and then you get rid of them? NO NO NO.
It's the same reason why people buy "expired domains". Simply because before search engines, directories & web sites have a chance to notice there is no web site anymore, someone simply changes the site to fit their needs and utlizes the traffic coming in!
Let's say that you've made up your mind and you want to change the entire layout of your site. Before you do so, go through all your links and record the page rank that your pages have right now.
Ex.
Page 1 (PR 3/10)
Link: www.yoursite.com/page1
Page 2 (4/10)
www.yoursite.com/page2
Page 3 (2/10)
www.yoursite.com/directory/page3
Now I would look at those pages and utilize the pages that have a rank of (4/10) for my most important information when I consider the new layout. Doing so will automatically get my page listed under the new information within Google.
Typically, I don't usually re-use pages that have a rank of 2/10, 1/10, 0/10. Anything higher will be strongly considered.
This will help search engines to recognize, a) Yes the site has changed, b) we are familiar with the pages they are re-using.
People BOOKMARK pages all the time!
What if you deleted all your old pages and created some new ones what will happen to your visitors that have a bookmark for those old pages??? They are now left feeling like your site is down, deleted or doesn't exist anymore.
If you're going to discard some old links, try adding a "Page no longer exist" message to let your users know that you know about the problem and you are sorry for the inconvenience.
So when re-designing your entire site, don't be afraid to re-use those high ranking links you already have for your site.
Searching For All Your Pages Online:
If you're like me, and you have well over 500 pages listed in google, you'll want to go through google to see if you've missed any pages while performing changes on your site.
type in "www.yousite.com" in Google so that you get your main link. Don't forget the brackets.
You should see underneath your listing a link that states:
"[ More results from www.smartads.info ]"
This will show you all the pages listed within Google. Take a moment to go through all the links found within Google and make sure that you've caught all your old, outdated pages and correct the problem accordingly.
Good luck with all your changes!
More Site Maintenance: 
