Extra time is only 30 minutes.So there's a strong chance that the scores will still be level after extra time if there is extra time.That would normally lead to penalties,where the underdog is typically less of an underdog than it is over the 90 minutes of normal time.
Supposing the best odds for normal time were 2.00 the favourite,4.00 the draw,4.00 the underdog. That's a 50% chance that the favourite wins in normal time,and a 25% chance of a draw. Now we split that 25% chance of a draw to get the percentages of going through.Maybe we give the favourite an extra 15% chance of going through,and the underdog an extra 10% chance. So we think the favourite has a 50 % chance of going through in normal time [ odds of 2.00],and a 65% chance after extra time [about 1.54]. The underdog has a 25% chance of going through in normal time [4.00],and a 35% chance after extra time [about 2.86].