I had this exact same problem with the retraining rules, both replacing old powers as you level, and retraining one ability every level. Both seem really silly to me. I posted on the Wizards website and got various handwaves for ways that I could justify this in-game, however, nothing has really stuck with me as a viable reason why a character suddenly loses access to burning hands as soon as they can cast fireball.
It would be fine if the skills system didn't work exactly the opposite, where as you level, you remember every easy skill check you ever learned, but for some inexplicable reason, everyone loses their "apprentice" level powers. It makes no sense. I can believe fireballs, I can believe elves and dwarves; I can't believe that heroes are forgetting how to punch as soon as they learn how to roundhouse kick.
This was the thread I started over there if anyone is interested:
http://forums.gleemax.com/showthread.php?t=1047821
...
Here was the most telling example of why I despise this rule:
"Hey, master, can you show me how to cast Burning Hands?"
"Sorry, kid, I forgot. Want to learn Black Fire, Acid Storm, or Icy Tomb, instead?"
"Nah, sorry, can't cast those yet."
*silence*
"Well, see ya."
"Catch ya laterz."