12AX7 or 12AT7

Can you replace a 12AX7 tube with a 12AT7
Yes and no, mostly no. It will plug in and work but it is not designed to be a replacement. The big differences are that the X has about twice as much gain and the T can handle quite a bit more current. So you usually see an X in the early stages to take advantage of it's gain and a T on the output or as a driver since it can source more current. The X also has quite a bit higher plate impedance. All these factors mean that a circuit optimized for the characteristics of the X probably won't sound as good with the T.

If you replace an X with a T, the gain is reduce and it may sound like crap probably but no harm will be done. On the other hand, if a T is being used toward the limits of it's design, replacing it with an X can push the X past it's limits.