However, the gilgul of the 'sparks' which were already rectified, they come into the body and have the status of an Ibur -- this means that they do not take part [or get affected by] any aveirot (sins) that this person does in this body; rather, the 'sparks' only take part in the merits (just like what happens with an Ibur of the Nefesh of a Tzaddik who has already passed on). We find that the sparks which were not rectified at all through the fulfillment of the mitzvot that are relevant to him, or through a serious aveira (sin) [the kind of sin where the person won't rise during techiyas hameisim] -- this person must reincarnate into a new body (which is now his body), and the sparks that he previously rectified through the mitzvot (even if they were damaged in a small sin) -- they come with him [into this new body], but with the status of an Ibur (meaning, they are not harmed by the aveiros he does in this new body, but they benefit from his mitzvot), even though the rectified sparks are coming into him in the form of a gilgul (meaning, they are joining him with his new body at the time of his birth).
So just as the 'sparks' (of the tzaddikim which come in the form of an Ibur while they are still alive) are not damaged or blemished by an aveira (because the sparks of a Tzaddik do not require rectification or gilgul), so too when they (his sparks from a previous gilgul where they [the sparks] have been rectified) can come into his body while he is still alive (to help him). All of this only happens if the person merits getting a Nefesh or a spark of the Tzaddik through actions. [You can see I am confusing where R' Chaim Vital is referring to sparks of a Tzaddik entering into a body versus his own rectified sparks which enter his body either through birth, and I think also when someone rectifies an aveira that he did - I believe the text is saying that those sparks which previously fell into klipah re-enter his body during his lifetime if/when he does teshuva.]