So a month or so ago, I installed Windows Server 2012 R2 Standard Eval. The server was just being used as a quickbooks server (remote desktop only used for administration, only runs Quickbooks DB software & file sharing)
The finance team likes the new setup, so it's time to buy a license, but I realised now that Standard, isn't the "base level" package, it's "Essentials", and from what I can tell, there's no REAL need for standard.
Please tell me there's some way I can just buy Essentials, and it'll downgrade Windows 2012 Server Standard to Essentials, without me having to rebuild the entire machine, migrate quickbooks, etc.
There is no way to downgrade, you will need to start fresh with 2012 Essentials media.
Using the DISM command, you can go up, but not down.
http://technet.microsoft.com/en-us/library/hh825157.aspx
No, there is no "downgrade" path from Server 2012 Standard to Server 2012 Essentials.
As noted in the comments, you might not want Essentials anyway, so check out the Server 2012 editions comparison chart before you go any further anyway.
I had to perform the same task.
I installed Server 2012 R2 Standard by mistake.
I already had the Windows essentials key label on the server. I just didn't see it.
To downgrade is simple:
Inside Windows, I run the installer of Server 2012 R2 Essential. During the install you will be asked:
It´s possible and simple. You have the choice to maintain the files or not.
It finishes in about 15 minutes.