The biggest problem, is that the 6922/6DJ8 only is rated at about 220V, while the 6N2/6AX7/6AV6 is rated at 330V. Also, the 6922 has FAR LESS gain than the 6N2... about a mu of about 33 for the 6922, as opposed to a mu of about 100 or so for the 6N2. This will upset the operation of most circuits.
To replace a 6N2, I'd be inclined to just re-wire the socket to accomodate a 12AX7 (it should only require moving 2 or maybe 3 connections for the heaters, that's all)...