How to upgrade windows 2012 edition with out media?
Using command-line utility, DISM, the Deployment Image Servicing and Management tool can upgrade server editions with out lose any data(A backup prior to making the change is always good) .
Note:If the server is a domain controller,demote the DC to a member server before you upgrade edition.)
The editions of Server 2012 fall into three groups. First there are the main editions, Standard and Datacenter. These are what most businesses will license.
Next there are the low-end editions aimed at small businesses, Essentials and Foundation.
Finally, there are specialist editions for specific roles. These are Storage Server and Hyper-V Server.
Read below blogs to know more
So next question,can i downgrade?
Answer is no,you may have to install from scratch.