Well impermanence is an aspect of Dukkha. So perceiving impermanence as being a part of Dukkha is a wise thought. I think it's a matter of also referring it to Dukkha all the time. It's not a matter of a change of the subject, because impermanence is an aspect of Dukkha. Because of impermanence there is Dukkha, so referring it always back to Dukkha, it remains a Dukkha reflection. As we refer it back to Dukkha, we may find that the perception of impermanence may change into something else like the impersonal nature of things, which is also an aspect of Dukkha. If we don't refer it back to Dukkha, because of impermanence there is Dukkha, it may be changing the subject and wandering off the main subject. But if we refer it to, "because of impermanence there is Dukkha," and refer it back to the subject I don't think it's going off the subject.