How do I select a new SMTP "Primary Server"?
I haven't figured out a way to rearrange the servers so that a different one is the Primary, but I was able to work around it by simply tapping on the Primary server entry and change the details so that it points to the server that I want.  That way, the one I am actually using is the Primary.

As Christian said in the comments, it doesn't matter, because if I only have one server actually turned on, that is the one that Mail will use.