I would assume mixing in buttermilk for milk would mess things up since it messes with the acid ratios, which is one of the things that really matters in baked

I would assume mixing in buttermilk for milk would mess things up since it messes with the acid ratios, which is one of the things that really matters in baked goods with chemical leavening agents. Usually something with only milk would use baking powder, whereas something with buttermilk would use some portion of baking soda.