Shlichus

In Chelek Yud Zain Lku"s the first sicha, the Rebbe speaks about the contrast, as related in a story of the Alter Rebbe, between the big Aleph of Adam (recognition of one's greatness) and the small Aleph of Moshe (a sense of Bitul). The Rebbe goes on to be meadayek in the Alter Rebbe's wording that Adam still had the advantage of a big Aleph (ac"p b'helem) even after the Chet Etz HaDas.
The Rebbe learns from here a deeper explanation (Ois Zayin) in V'amech Kulam Tzadikkim that we all have this level of Adam HaRishon inherited to us. And therefore even if we were once in a situation the opposite of good (similar to Adam HaRishon l'achrei HaChet), but now we are involved with Torah and Mitzvos we are considered to be doing the Avoda of a tzadik (similar to Adam who after the Chet still had the big Aleph (according to the diyuk in the Alter rebbe's words)). And the Rebbe then contrasts this person with one who has the status of a baal Tshuvah that still needs a shmira me'ula shleo yachzor l'suro even after he is living a Torah life.

Now this whole paragraph seems to be without source, in that it is original, it is the Rebbe's vort. It seems to somewhat fit a gaang of the Rebbe's that even if one is not necessarily holding by a high madreiga it is still incumbent upon him to fir zich, accord himself, with behaviours and hergashos at times that are higher than his level e.g. the Alter Rebbe's deveikus of "ich vill nor dich alayn" and others.

A question that comes up is, what is the difference between the ball tshuvah at the end who needs a shmira me'ula and the yid in the beginning who once did not good things, like Adam once did, though now that he is engaged in Torah and Mitzvos is at the darga of a Tzadik with no connection to evil?

Yesh Lomar, that perhaps the difference is a baal tshuvah who is on shlichus. Maybe there is even a halchaik question if a baal tshuvah is allowed to go on shlichus? Since he needs a shmira me'ula maybe he shouldn't be going into such circumstances where he is surrounded by his former lifestyle. And the Rebbe writes that since he is engaged in Torah and Mitzos he is on the level of a tzadik and doesn't need all of these precautions.