If that's a definition of morality, then morality is a subset of psychology, which probably isn't what you wanted.

If that's a valid argument, then logic, mathematics, etc are branches of psychology.

There's a difference between changing your mind because a discussion lead you to bound your rationality differently, and changing your mind because of suggestability and other forms of sloppy thinking. Logic and mathematics is the former, if done right. I haven't seen much non-sloppy thinking on the subject of changing preferences.

I suppose there could be such a thing -- Joe designed an elegant high-throughput gas chamber, he wants to show the design to his friends, someone tells Joe that this could be used for mass murder, Joe hadn't thought that the design might actually be used, so he hides his design somewhere so it won't be used. But that's changing Joe's belief about whether sharing his design is likely to cause mass murder, not changing Joe's preference about whether he wants mass murder to happen.

Are you saying there has never been any valid moral discourse or persuasion?

No, I'm saying that morality is a useless concept and that what you're calling moral discourse is some mixture of (valid change of beliefs based on reflection and presentation of evidence) and invalid emotional manipulation based on sloppy thinking involving, among other things, undefined and undefinable terms.