So basically you're taking the "empty slot" rules from 3x and removing a line from the text?
We've always done that. Replacing a memorized spell takes 15 minutes. Don't have knock memorized, wait 15 minutes and lose the previous memorized spell and you're good to go.
d20 said:When preparing spells for the day, a wizard can leave some of these spell slots open. Later during that day, she can repeat the preparation process as often as she likes, time and circumstances permitting. During these extra sessions of preparation, the wizard can fill these unused spell slots. She cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because she has cast a spell in the meantime. That sort of preparation requires a mind fresh from rest. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if the wizard prepares more than one-quarter of her spells.
We've always done that. Replacing a memorized spell takes 15 minutes. Don't have knock memorized, wait 15 minutes and lose the previous memorized spell and you're good to go.