[quote]Can I switch the platform of my preorder from xbox 360 to ps4?[/quote]
Yes. You need to [url=http://support.activision.com/articles/en_US/FAQ/Retailers-Offering-Destiny-Beta/]contact your retailer[/url] to make this change.
[quote]will I keep the benefits I got from playing the beta?[/quote]
Only if you play on the same gamertag that you played the beta on.