This is a concept I just don't like. People don't just forget how to do things so they can do new things.
The way I handle it in my current 3.5 game is giving the Sorcerer ways to make his known lower level spells more useful at higher levels.
One way this can be done is by using spontanous metamagic feats.
For example the spell sleep is good for a low level sorcerer , but at medium level its no good, because its hit dice limited. By allowing the Sorcerer a bonus power that increase hit dice limit by 50% , you increase its use at higher levels.