As everybody knows, Actions in the form of papam and punyam are accumulated birth after birth, and goes by the name sanchita karma.
A small part of sanchita karmas which are about to give rise to result needs a birth. So a fresh body is taken to experience this set of karmas, called prarabdha karma.
Among these 2, those actions, that are still in the bank account in the name of sanchita cannot be averted at all by any chance. It would certainly give rise to its result in a future body when those sets of actions mature to give the result.
In the same way, that body which has been taken only to experience the effects of actions done earlier but matured now as prarbdha, is also cannot be averted at all except undergoing them in toto.
Prayaschitta karmas are those specific karmas done as an expiation to avert the evil effect that gonna rise in the future for doing a specific act now. So, unless we know what wrong act is we have committed, we can't take up prayaschita karma related to it. So, it also shows that the scope of prayaschitta is limited to this birth only, for we don't know what acts we have done earlier. Also, we do know what wrong karma we did for which the shastras give us an option to the avert the evil that's going to mature in future.
So, prayaschitta karma can't avert past actions, sanchita karma, that can't be averted at all without producing fruits just like prarabdha.
Now, a question may arise as to how else will all the earlier sins done in the past lives and also the karmas doing in this present birth which will mature in some later births called agami karma, be averted? Because without all the karmas, both papa and punya being purged completely, there is no scope for release from birth and death. So there is no release from the misery called samsara.
For this, the answer is : The only way to avert all sanchita karma and agami karma is to attain gnyana. Knowledge of the self. In short, knowing that i'm not the agent at all. Not just now, but for ever.
In Gita, Lord krishna says. For gnyana burns up all acts. It burns up all karmas, sanchita and agami, but not prarabdha. - See shankaracharyas bhasyam, alladi mahadeva shastri, ghambirananda's or V.panoli's.
Not only that, Among the purifier knowledge is the supreme.
Hope this helps.