Retail product (FPP) come with limited downgrade's right wrote in each EULA.
For Windows Server 2012 R2 Essential. (http://download.microsoft.com/Documents/UseTerms/Windows%20Server_2012%20R2%20Essentials_English_786e41e5-0113-45ee-bbc3-536624460b15.pdf)
It's wrote that way:
DOWNGRADE. Instead of creating, storing, and using the software, for
each permitted instance, you may create, store, and use an earlier
version of Windows Small Business Server 2011 Essentials or Windows
Server 2012 Essentials software. This agreement applies to your use of
the earlier version of Windows Small Business Server 2011 Essentials
or Windows Server 2012 Essentials that you elect to use under this
downgrade option. For the avoidance of doubt, by electing this
downgrade option: (i) you will not have the right to create, store, or
use a greater number of instances of the software than are permitted
under this agreement, and (ii) you will need to acquire licenses for
the number of processors in the physical server in accordance with
Section 2 of this agreement. If the earlier version of Windows Small
Business Server 2011 Essentials or Windows Server 2012 Essentials
includes different components, not covered in this agreement, the
terms that are associated with those components in the earlier version
apply to your use of them. Microsoft is not obligated to supply
earlier versions to you. At any time, you may replace an earlier
version with this version of the software.
The limitation in your case is you can go only N-1, not N-2 like on VLSC. As such please recall the licensing please. Explain your situation and they can give you a onetime activation code.
Downgrading a OEM/FPP(retail) is always a pain to do, the problem with such downgrade is that they assume you already have a working product key of the older version, so technically you never have to call them for a activation code, as you can use the older activation code and keep the one you bought not used.
She suggested you a VLSC license because in the portal they are easy to downgrade, you got a direct access to them. In your situation, as the eula cover it, it's a bad advise. (and for the ten copy, I wonder if she think for CAL use)