[Assume that I have a site with 3 years old and I have pr5. Now I’m going to entirely change the site design as well as different content I have put compare to earlier. Assume that I had a social bookmarking site, now its a directory. Google will give pr for the page. Is that ok?. Then why my page rank is pr5 still? I have changed the page. So page rank will be given to domain name auh? I’m something confused.]
PageRank isn’t assigned to domains, it is assigned to specific individual URLs. It isn’t based on the content of your page, it is based in the inbound links to your page. Changing your page’s content generally will not affect the PR of that page, however changing URL structure will.
That is not entirely accurate. Real PageRank is calculated at query time and uses the current link graph. So real PageRank is effectively updated immediately following the update of the link graph.
It is only the Google toolbar data that gets infrequently updated. Toolbar PageRank isn’t your current PR it is only a snapshot of what it was some time in the past.