They are interchangeable, just the ECC82 will be a have slightly less gain.
The voltage gain of a valve is entirely dependent on the circuit in which it operates. Anode voltage (max 250v in both cases) and the anode load primarilly set the gain. I don't remember seeing voltage gain published as part of a valves characteristics.
check out this from Wikipedia "vaccum tubes"
"...The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the overdrive that tube amplifiers exhibit at high volume levels (known as the tube sound). To remedy the low-volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or standing current. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called grid bias."
If the circuit in which the OP swapped the valves was carefully designed to work the 83 optimally (particularly re. harmonic distortion) then there is a very good chance that the pre amp will not operate as it should.
But there is also the chance that under very small signal conditions it could be ok. In any event, no harm will be done to the device