First of all, european airlines don't practice the standby principle common to american carriers.
The *G priority waitlist concept is when you book/rebook your ticket and the booking class is full, you get on a waitlist. If additional seats become free in that booking class, you would jump the queue.
If you have a dirt cheap ticket which is booked in a fare that doesn't allow changes, you have to buy a new ticket.
The only time I have seen SAS allow you on an earlier flight is when the original flight was hopelessly over booked. None of the flights on Aug 26 look like that:
SK 502
LHR
26/08/07 10:35 AM CPH
26/08/07 1:35 PM 321
Su,M,T,W,Th,F
67% / 19m C4 D4 J4 Z3 Y9 S9 B9 E9 M0 H0 Q0 V4 W0 U0 K0 L0 T0 G5
SK 504
LHR
26/08/07 2:05 PM CPH
26/08/07 4:55 PM M80
Su,M,T,W,Th
54% / 20m C9 D9 J9 Z9 Y9 S9 B9 E9 M9 H9 Q9 V7 W9 U2 K0 L0 T0 G9
SK 506
LHR
26/08/07 4:50 PM CPH
26/08/07 7:45 PM 321
Su,M,F
42% / 28m C9 D9 J9 Z9 Y9 S9 B9 E9 M9 H9 Q9 V9 W9 U9 K0 L0 T0 G9
SK 1516
LHR
26/08/07 6:05 PM CPH
26/08/07 8:55 PM M80
Su
52% / 29m C9 D9 J9 Z9 Y9 S9 B9 E9 M9 H9 Q9 V9 W9 U9 K0 L0 T0 G9
SK 1508
LHR
26/08/07 8:10 PM CPH
26/08/07 11:05 PM 321
Su,M,Th,F
46% / 23m C9 D9 J9 Z9 Y9 S9 B9 E9 M9 H9 Q9 V9 W9 U9 K9 L1 T0 G9