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Your equivalence is rather silly since both sides are constants.  More completely:

A and &not;A = 0<br>
A nand &not;A = 1<br>
A or &not;A = 1<br>
A nor &not;A = 0<br>
A xor &not;A = 1<br>

You can just as well ask when XOR becomes NAND, or when AND becomes NOR, since they have the same values in the cases above.  Or since

2 + 2 = 4<br>
2 * 2 = 4

when does addition become multiplication?

Finding an instance where the result of two different operations happens to result in the same value doesn't mean much at all.