How some old website get new domains with higher PR?

Hi!
I saw that few websites changed their web site main address after Google changed their page rank. For example if they used "example.com" domain when they start their website, but after Google changed their PR they used another domain name for example "example.net". But both domain age is same. So how do they do it. I don't think they used two websites?

thanks!

 

interesting.. i also want to know.

 

ya plz tell me if u know