I guess my question is why do you have to down-grade if this is a Test box?
My understanding is that you licence Production Servers and as part of your systems Development/Testing you are likely to have other Servers for, say, System Test, Integration Test, Stress Test, User Acceptance Test, etc., as part of your Change Management for applying changes (including SQL Server Service Pack testing) to Production - your environment may be different depending on the estate size, maturity, processes of your environment.
With this, for some areas of testing you should have installed the same SQL Server Type and Edition, except when pushing through and testing an imminent change to be promoted to Production, and in which case you are allowed the Test environment as part of the Production Licence as long as the Test Server is never used for production purposes, including reporting, etc.
Anyone, please correct me if I'm wrong as I haven't looked at licencing recently for changes...and we all know MS love moving the goal-posts
Edit: corrected spelling error