Can 12AT7 be a substitute for 12AU7 tubes?

Is it safe to use a 12AT7 as a substitute for 12AU7?
I wouldn't think so. The 12AT7 with a mu factor of 60 can be used as a replacement for a 12AX7 with a mu of 100. Probably too high for a 12AU7 with a mu factor of 19 though.
In most circumstances I would say No .... I would ask the manufacturer of the component in question.

The tubes are electrically pretty similar, so you won't smoke anything, but as previous posters have noted the gains of the tubes are quite different, so there would be sonic change.
I've done the swap of 12AT7 in place of 12AU7 and with the tube gear I had the difference was pronounced. I definitely had higher gain and didn't have to turn the volume knob up on my preamp as I did when I had the 12AU7 tubes. The sound also was a little more forward but I had a little more detail. This was in my system though. With the gear you have you may hear something entirely different. Since I like to turn it up I preferred the 12AU7 tubes in my situation.
Thank you for all your responses. I called the manufacturer, they said I can use the 12AT7, but it won't sound right.
You should stay with 12at7. They are designed to increase current in the amp compared to 12ax7 and 12au7 which are designed for volume amplification. Mu doesn't matter in a lot of cases when it comes to 12at7 since a lot of circuits where they are used do not amplify sound but provide more current to output tubes - thus the increase of current.

See my review here: