920+ vs HDD extension? Owning a 718+ I'm pretty happy with what it can offer. And I'm not even using it at full. 

Came to concern that perhaps I can't change 1 HDD out of 2. Yes I have a backup via USB.  
Another point is upgrading HDDs. I guess its not possible like in 4 bay - just swap one by one without info loss, other things being equal.  


what makes  better sense - buying an extension or upgrading to 920+?