Nifft
Penguin Herder
Also, the Ambitious Effort ability (the one you must be thinking of, the one that gives you an extra trained encounter power from your class) says that it can be retrained at 21st to chose a higher level power, so I'm not sure that you can retrain that one willy nilly either.
The 21st level sentence could be interpreted to say that when you retrain, your choices have expanded (from 7th or lower to 13th or lower).Compendium said:At 21st level, you can replace this power with an encounter attack power of 13th level or lower from your class (instead of 7th level or lower).
Or it could be interpreted to mean that the only chance you get to retrain is 21st level (and the level increase is just a side note).
I can't find one reading more supported than the other.
Cheers, -- N