First off, I'm using "epistemic and instrumental rationality" as defined here:

http://lesswrong.com/lw/31/what_do_we_mean_by_rationality/

If you don't believe objective morality exists then epistemic rationality can't be applied directly to morality. "This is the right thing to do" is not a question about the "territory" so you can't determine its truth or falsehood.

But I choose some actions over others and describe that choice as a moral one. The place where I changed my mind is that it's no longer enough for me to be "more moral than the average person". I want to do the best that I can, within certain constraints.

This fits into the framework of epistemic rationality. I am essentially free to choose which outcomes I consider preferable to which. But I have to follow certain rules. Something can't be preferable to itself. I shouldn't switch your preference and back just by phrasing the question differently. And so on.