I am not sure exactly how they are doing it now on PMCO planes, though for UA metal this is nothing new as anything less than row 21 would be E+. Eventually this will be the way it is on the entire fleet. I wouldn't be surprised if they are already blocking the seat maps in case there is a plan switch. Also keep in mind that 2Ps (silver) can not book E+ until 24 hours before departure without paying for it as part of the new rules.