Suppose acting out of concern for the morality of my future selves was moral.

For a reductio, assume moral motive was sufficient for moral action. Suppose you self-modified yourself into a paperclipper, who believed it was moral to make paperclips. Now, post-modification you could be moral by making paperclips. Recognising this, your motive in self-modifying is to help your future self to act morally. Hence, by our Kantian assumption, the self-modification was moral. Hence it is moral to become a paperclipper!