Say for example you own a website (domainA.com) that ranks very well in the search engines for some good keywords... You then transfer the entire site's content to a different domain (domainB.com). You then simply redirect domainA.com to domainB.com... DomainA.com will lose all of its SERPs right?
Yes may be not in some condition if u redirect ur trafic after 10 to 15 seconds delay to domainB.. With metatag refresh code.
If you do a 301 redirect, you keep your pagerank http://www.webconfs.com/how-to-redirect-a-webpage.php
NO, if you copied the entire site from domainA to domainB with the same URL's or even applied a page to page redirections and used L301 all the PR values will be passed properly then the current site ranking in SERP's will be replaced with the new site and URL's. However it will take some time to get rid of the old site and replace it with the new one depending on the current site value and linking quality After all try to contact the linking websites and ask them to replace the links to point to the new domain.
Yes. you will lose the SERP's. and also google may consider SiteB as a spam site as it has same content (duplicate content) like the first one.
When I moved my site, I got my pagerank 3 back in two weeks ... It's a wordpress blog and I imported all my posts from one blog to another. Then I deleted the posts from the old blog - actually, I just put links in each post to the corresponding post in the new blog. Put a 301 redirect on the old blog which tells Google that it's a permanent move and it's good to go. No loss of traffic or SERPs or Pagerank.
You need to do the 301 properly to get the positive effect of redirection, since it take time for Google to update indexes & GWT data it take some time to show the same results as earlier. Once you've done redirection properly I would suggest to use sitemap which will help you to index faster.