By the core rules, like they say, no you can't un-know a spell/power.
But, IMC I allow "upgrades" for free, but the DM has to agree that the spell is clearly an upgrade. Same school, same damage type, same save type, and so on.
For example, if you took Change Self, you couldn't upgrade to Alter Self (even though Alter Self's short description compares the two) since Change Self is a glamer and Alter is a transmutation. However, you COULD upgrade Alter to Polymorph Self, freeing up a 2nd-level known slot.
See, the problem with saying "no upgrades!" is that if your campaign doesn't start at first level your characters turn out completely different. A first-level Sorcerer might take Mage Armor, but want to drop it once he gets high enough to get Bracers of Armor. You could require a Feat to let people upgrade, but it's still unbalanced. If you create a level 15 Sorcerer it's easy to say "oh, I never would have taken that spell in the first place" to get out of taking the Feat, but it looks a lot different when you're low-level.
I also allow a Sorcerer to "know" a metamagicked spell if he has the correct metamagic Feat. So, I could use a 5th-level slot to know Empowered Fireball to get around the longer casting time (but it's still a 3rd-level for purposes of save DC, etc., and it doesn't allow you to cast without that metamagic) These, of course, would count as an "upgrade" so you could un-know the basic Fireball.