The Rishonim ask how the process of teshuvah can undo the past -- e.g. the Sefer haIkkarim writes that if you smash an object into pieces, no amount of regret will cause the object to reconstitute itself, so how does the regret of tshuvah cause our past mistakes to be wiped away?
I saw a comment of the Sefas Emes (in the likkutim at the end of the sedfer) that perhaps opens a new approach to answer this question. The S.E. quotes Rashi's famous statement that Hashem's judged Yishmael "ba'asher hu sham", based on his conduct up until that historical point in time, irrespective of his future actions or those of his descendents, which were undoubtedly already known already to G-d. The S.E. extrapolates an amazing chiddush. From Hashem's perspective past and future are one and the same. If Hashem chooses to ignore the future and judge a person based only on the facts of the moment, the same principle should dictate that Hashem ignores the past and judges based only on the facts of the moment! So what are we klapping al cheit for?!
The Sefas Emes explains that events of the past leave a roshem, an impression, a mark that is part of who we are at the present moment. We are not judged on past events, but we are judged based on the roshem that remains and is part of who we are at the present moment.
Based on this, I would suggest that tshuvah need not undo the past -- the question is misplaced, as the past has no bearing on how Hashem judges us. What needs to be undone in the roshem past events have had on our neshomos, a roshem that is part of our existance in the present but which can be cleaned though the process of viduy and charatah.