Вот еще что англоговорящие об этом думают:
"Well taken literally the few words are
Nitwit-stupid
Blubber-fat
Oddment-leftover or odd
Tweak-To change something slightly in order to improve it or to pinch and pull with a sudden jerk and twist or to bother
As per the essay @DVK posted a link to he could have been making a comment about each of the 4 houses.
Personally I think that it could be read as a sentence "Nitwit, blubber, oddment, what ever you were before will change""

и еще:
http://www.theprospect.net/dumbledores-advice-on-college-nitwit-blubber-oddment-tweak-6957