First of all, the 12AX7 filament pins' wiring would have to be reconfigured, as described below. If you just pop one into a regular 12AX7 socket, one of two things will happen:
Best case, nothing happens, as pins 4 and 5 are connected together for 6.3 V filament operation. Filament stays room temperature cold. Just plays "possum".
Worst case, if the 12AX7 has 12V filament power, the 6N2P will light up very brightly, for a very brief period of time, until the filament burns out.
The 6N2P has the same base pinout as 6DJ8, or 6N1P, filament is pins 4 and 5, and pin 9 is an internal shield , so it would
not be a "drop-in" replacement for 12AX7.
It is roughly similar to 12AX7 in its basic operating characteristics, mu (gain) is roughly 100, transconductance and plate resistance are somewhat different than the 12AX7, but not so different, that the basic circuit topology and resistance values don't need to be changed. It comes closer to the op chars of the 2C52, which is a mil-spec 12SL7 variant, but with higher gain than 12SL7..
There IS a such a tube as a 6AX7, but these are not very common, they have the same pins 4 and 5 filament, 6.3V requirement.
I once bought a "flat" of 100 6N2Ps, about a decade ago, and ran them through my Hickok 539C (transconductace) tube tester, they varied over a range of approx. +/- 20% worst case. Still very usable, but not entirely suitable in cases where reasonably accurate tube matching would be required. These would be suitable for guitar amp usage, not so much for "audiophile" purposes. 6N2P, some folks swear by them, and some folks swear
AT them
![Cry :'(](./images/smilies/cry.png)
YMMV, etc...
HTH
/ed B in NC