Sirius:
The engine power available is based on a minimum specfication engine - the power that the engine should always be capable of producing, even 1 minute before reaching TBO.
In flight testing, the airframe manufacturer will know this value from the engine 'deck' provided by the engine manufacturer, corrected for any installation losses due to the airframe. Since the engine provided is probably above this specification, a way to correct the data is made.
For some Digital Fuel Control engines, this is a simple process of adjusting the maximum power via the flight test computer to that which would be provided by the spec engine. For hydro-mechanical fuel controls, things are a bit more complicated...
It's not a 'new' engine that is used, but the minimum specification one.