I switch between brands all the time with my two, but it's all high quality grain free kibble so that might make a difference. They have zero reaction to the switches, when I'm done with one bag I just move onto the next with no problems.
I vaguely remember that I did mix the two together to start out, but I did that less and less when I saw no differences in them, their poops, their general demeanor. So much easier to not have to do the switch gradually. Thanks dogs!!