You can upgrade, we have the easy plus and there is an option on the online portal to upgrade to the premium for $20 a day, I think i'm right in saying though that the easy plus covers more than the well spirits, its the easy one that only cover well.
In regards to paying the difference, I think that is the policy now. It didn't used to be, if you wanted a more expensive drink you paid for the full cost but I have read that that changed several months ago.
We are yet to sail though so this isn't first hand experience, just what I have read.