Specification of SuperMemo 2
$EF$ is the "easiness factor". This means that the greater easiness you have, the longer will be the interval to next review ($I_\text{new}$).

So if you will use the old $EF$, the new interval value will not take into account your last review quality, which makes less sense IMO.

For the other hand, as I can see in the original implementation of algorithm, they calculate the interval first and then they calculate the easiness:

procedure Repetition(ElementNo,Grade:longint;var NextInterval:longint;commit:WordBool);
  var DataRecord:TDataRecord;
  begin
    DataRecord:=GetDataRecord(ElementNo);

    with DataRecord do
    begin
      if Grade&gt;=3 then
      begin
        if Repetition=0 then
        begin
          Interval:=1;
          Repetition:=1;
        end
        else if Repetition=1 then
        begin
          Interval:=6;
          Repetition:=2;
        end
        else begin
          Interval:=round(Interval*EF);
          Repetition:=Repetition+1;
        end;
      end
      else begin
        Repetition:=0;
        Interval:=1;
      end;

      EF:=EF+(0.1-(5-Grade)*(0.08+(5-Grade)*0.02));

      if EF&lt;1.3 then
        EF:=1.3;
        NextInterval:=Interval;
      end;

      if commit then
        SetDataRecord(ElementNo,DataRecord);
      end;


The same is in the specification: they calculate next interval at step 3 and $EF$ is calculated at step 5.

I'd like to understand what all these magic coefficients mean and how they were found, but unfortunately didn't find this information.