I agree that you should be fine in the long run. You may see a slight dip in the beginning, but this should balance out and rankings will return. Make sure you apply the 301 re-direct from old site to new. That is the most crucial step. Without that, you really will be starting from scratch.

You can find Google’s webmaster guidelines on 301′s here: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=93633

good luck with the new site. I hope it goes well.

I would not recommend changing your domain unless your original domain is completely irrelevant. Google puts a lot of weight on the age of a site – the older the better – so yes, you would be starting from scratch with a new domain.

You could always forward yoyr old domain to your new domain if you have loads of existing traffic.

I use a product called SEOPressor for all my SEO work in WordPress. Here’s a review of it;

http://www.squidoo.com/honest-seopressor-plugin-for-wordpress-review

Google also put a lot of weight on the relevancy of the domain name. For example if your site was related to dog training, http://www.dogtraining.com would work a millions times more than something wacky such as http://www.quangomanneria,com, or some such nonsense.

Unless you are called Yahoo, which is a great name…..