I wouldn't do it; each tube is different electrically; the 12AX7 has higher gain but the 12AT7 has higher current.
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Some circuits can use both.. Also the lowest gain of the series is the 12Au7's but make sure you can use any of this family first..
Beyond that 12AT7's for the most part are easier to find in better quality these days, even some of the new productions are pretty decent..
12AX7s are higher gain, which also in many applications equals much more Noisey but un-needed cause you can just turn the volume up a little on a AT7 and get better sound and quiter background.. This is just in my experience thats all.