GOOD QUESTIONS!
So far as I know no announcements have been made.
I suspect seat selection will work the same way it does now. I am pretty sure that if you purchase a BS seat you will get in the front, like you do now.
I suspect that Elite upgrades will be the same confused chaotic situation we have now, only worse. I carry my Elite Card with me just in case.
I don't think the 4 credit upgrades were ever available for tickets booked on Southwest.com, but I never tried.
I also never tried to upgrade at the gate. I wonder if that is or will be offered to people up purchase on Southwest.com. Southwest does have a $40 upgrade to A1-15 which could be applied to Airtran passengers with a Biz seat.