This question, rhetorical though it is, and its answer are an illustration of something, namely of the difficulty in changing. For us, it may sometimes be impossible. Many changes are, in fact, impossible to make without help. And that is what I hear God getting at in this verse.
I am tempted to go off on a tangent about a lot of minor details, but the crux of this verse; the pivot on which it turns is how difficult; how impossible it is to go from habitually doing evil to habitually doing good. Inevitably, the objection will be raised that people have done precisely that: gone from doing horrible things to doing wonderful things. This objection may be valid, but only if God was not involved in the change process and the person’s motives were pure. If God is involved, the impossible becomes probable. If the person’s motives are still evil, then that individual has not changed what they do, only what it appears to be to the rest of us. God looks at the heart; at what drives a person.
The Bible is quite literally brimming over with this message: I cannot do good with good motives without God’s help. Paul would later write that it is God Who works in us to will and to work His good pleasure; it is God Who gives us good motives and the wherewithal to follow through on those motives with good actions.
Can a leopard change its spots? With God, all things are possible. Can I do the right thing for the right reason, even if I have been accustomed to do evil? With God, all things are possible. Without God, no spots are changing and no good is taking place.