The only times you can replace lower level powers with higher level powers are at the times stated on the chart on pg 29 of the Player's Handbook. Those are 13th, 15th, 17th, 19th, 23rd, 25th, 27th, and 29th level. Otherwise, you can only use retraining to train some power of the same level or lower. That's straight from the description of retraining.