People never change.
In some respects, yes. But small, incremental improvements can be made. And sometimes major conversions can take place. Think of St. Ignatius of Loyola or many other saints.
Actually, great falls are possible as well. On CathInfo someone posted that "Jehanne", a former CathInfo member, is now a rabid atheist with a blasphemous blog. Another family (all three members of this family were on CathInfo) gave up the Faith -- a baby boomer aged single mom with son and a daughter. First the son and daughter gave up the faith, then she followed suit. She is in a nursing home now.
But I agree that even when you vastly improve yourself, your old self is sometimes just waiting under the surface ready to break out again. It doesn't matter what vices that "old self" had: uncharitable, worldly, wrathful, undisciplined, immature, impure, alcoholic, etc. That's why we all have to be vigilant until the day we die.
As I said in another thread, there ARE vices that each man doesn't have to worry about, because of developed virtues, background, personality, etc. For example, sodomy. Many men don't have to waste any energy fighting this sin. But vices that were very much part of you just 10 years ago? That's another story.
So "in theory" we are each capable of all sins, from the greatest to the least. That is, if God withdrew all His grace all at once (which isn't likely to happen!) That's well and good, in theory. But we are MUCH MORE capable, and likely, to fall into tendencies from our own past.Long story short, even though those tendencies might remain under the surface, you can smother them with virtue. You can make an act of will that you're not going to be your old, ______ self. We have free will, so we get to decide if we're going to be good or bad, at any given point. We can merit heaven or hell based on our own free choices.
We're not Calvinists who think that some men are "born bad, destined for hell" and others are "born good, destined for heaven".