Is there a Torah idea that one can atone for future sins?

Our Gemara on Amud Aleph discusses the reason why the Nazir must bring a sin offering. Though there have been numerous Midrashic and exegetical interpretations, the most provocative one is offered by Ramban on Chumash (Bamidbar 6:11). He says: 

In accordance with the plain meaning of Scripture, [it is because] this man sins against his soul on the day of completion of his Naziritehood; for until now he was separated in sanctity and the service of God, and he should therefore have remained separated forever, continuing all his life consecrated and sanctified to his God.

So according to the Ramban, the real sin of the Nazir is stepping down and leaving a life of intense sanctification and abnegation and becoming an average citizen.  

Rabbenu Bechaye (Ibid 6:13) finds this Ramban difficult to understand.  He asks, when do we ever find examples in the Torah where a sin-offering is given in anticipation of a transgression?

What exactly is bothering Rabbenu Bechaye?  It would seem that he cannot accept the idea that the Torah would expect a sin offering to be for a future action.  Even though we may say that the Nazir, in his head, has presently decided to return to normal engagement with pleasures of the world, he has not DONE anything yet and should not be held liable for his thoughts.

We can defend the Ramban with a similar precedent found in Gemara Niddah (31b):

שאלו תלמידיו את רבי שמעון בן יוחי מפני מה אמרה תורה יולדת מביאה קרבן אמר להן בשעה שכורעת לילד קופצת ונשבעת שלא תזקק לבעלה לפיכך אמרה תורה תביא קרבן

The students of Rabbi Shimon ben Yoḥai asked him: For what reason does the Torah say that a woman after childbirth brings an offering? He said to them: At the time that a woman crouches to give birth, her pain is so great that she impulsively takes an oath that she will not engage in intercourse with her husband ever again, so that she will never again experience this pain. Therefore, the Torah says that she must bring a sin offering.

It would seem that here she is bringing an offering for future actions, i.e. she will violate the oath once she recovers from the rigors of labor.  However, this is not the case, as the basic interpretation of the Gemara is that she is liable for making a null oath, that is an oath that cannot be fulfilled. (See Tur Ha’Arukh Vayikra 12:7) This can be compared to a person who makes an oath that he will not sleep for three days.  The Gemara rules (Nedarim 15a), he may sleep right away as it is an impossible and null oath so it is inherently false, and also is liable for lashes as he made an invalid oath and used God’s name in vain.

However, it is possible that Ramban understands the Gemara Niddah (31b) differently, and indeed he may see this as a proof that sometimes the Torah holds a person liable for a particularly improper intention and future act.

There is another possible answer.  The Ramban sees the sin of the Nazir as occurring right now, even though he has not yet tasted a sip of wine.  By the fact that he is ready to engage in indulgence, he is now already on a lowered status and his sin begins now.  Even though it is mostly in thought, it has some basis in action, because he has taken the steps of shaving and the ritual process of declaring his ending of Nazirhood.