We are now into the very esoteric realm of what are called separable verbs in studying German. There are very few such verbs in English, and I do not even know the correct term for them in English.
He changed about X
means that his current attitude or opinion about X is different from his previous attitude or opinion. It reflects a difference in an internal mental state.
He changed X about
means that he rearranged X and reflects a difference in something external.
More generally, "what changed about X" is asking for a description of the change; "what changed X about" is asking for a cause of the change.
Thus, your second example sounds a bit strange, at least in American English, because it is not completely clear which sense of "change about" is intended. That sense is typically clarified by the placement of noun or pronoun.
I can't see how he changed
I can't see what changed about him
mean the same thing and are equally idiomatic in American English.