Unless you have the "slow start" upgrade with the 5670 tube, the 6SN7 is not easy to substitute. And if you DO have that upgrade, there is 6.3v between pins 1 and 9, not 4/5 and 9.
In addition to the different pinout, the upgrade board has an adjustable driver bias which allows operation of tubes with different characteristics. The v1.0 Paramount with the 12AT7 does not have this feature. And the 12AT7 won't work with the upgrade board, the adjustable bias does not go low enough.