I heard this joke the other day, and sadly - it reminded me of Google.

An 80 year old woman had a vision one night, she saw and spoke to God. She asked him, "how much time do I have to live."

He said, "you have 30 years left."

So that whole year she had a ton of cosmetic surgery, she had a face lift, a tummy tuck, her nose reshaped, liposuction, she completely did herself over. She figured as long as she was going to live another 30 years she was going to look young again.

After all this was done, that same year she was crossing the road and was hit by a car - killing her instantly.

When she arrived in heaven she walked over to God and said, "What happened? I thought you said I had another 30 years."

God replied, "I DIDN'T RECOGNISE YOU"

Now, not wanting to imply a relationship between God and Google but this is sort of what can happen to you when you redesign your web site.

You see Google likes established sites that it can trust.  New sites need to establish this trust for a period of time before they can earn rankings.  To get around what has been called the "Google sandbox", some people started to buy existing web sites and completely replace the content.  So the "new" site gains the trust and existing credentials of the previous site. 

Google responded to this approach by taking the view that if a large percentage of a web site was changed - then how can it continue to trust the site as it is practically a brand new site... in other words, it no longer recognises you!   If this happens, your site can be dumped into the "Sandbox" again - at least for a short period to make sure that you are credible.

So be careful when you are going through a site face-lift - make sure that there is still some content, structure, and similarity to the previous site or you may find that you are like the 80 year old woman in the joke and your rankings are killed instantly because Google no longer recognised you.