WP 8.1 official update

Yes. If you are currently running the developer preview, you will be able to update to the official 8.1 when it's released for your handset/region/carrier. You don't have to do anything special to make this happen.
 
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Around 2 million people has asked your exact same question, most of them even got answers.

Reluctantly though, Yes they can - whenever Microsoft/Your carrier decided to flip the switch and let us download.
 
It means no different between wp8 owners and wp8.1 preview owners to get official update ?

Correct. People with WP8.1 preview will get the official update at the same time as everyone else (and will get manufacturer firmware (Cyan for Nokia owners) at the same time).