I guess it's just me, but explaining your change as being caused by having leeway doesn't sound like she changed, just her circumstances. That's like saying;
"I passed by lady with a posh purse. If it was me yesterday, I'd have snatched and emptied it with utmost efficiency, but since my shoelaces were stolen, I didn't rob her. Am I civil now?"
The thing confusing me about this is, I can't tell if this discrepancy is good writing for Serena, or an accident/oversight on the author's part.
EDIT: For clarity, I'm saying she didn't, in fact, change. Not as a person, at least. Just her actions. Anyone will change how they respond to things when subjected to a change of environment.