AIX's MIXCOLUMNS AND ADDROUNDKEY operations can be switched?

If you see the pseudo AES code (you can have the whole description here in the link ) you'll see that addroundkey is used 3 times while the mixcolumns only once in the rounds

By the nature of mathematical operations, if you reverse the two operations (being respectively a multiplication and a sum of the polynomials), the result will be different since they are not linearly interchangeable operations. You can eventually exchange but in this case you would have to use an adapted polynomial. (and more mathematical operations, it would not be recommended)