Good question!
I will try to answer in the point format. Some people may want to read the post, “What Reincarnates? – Concept of a Lifestream” to get the background material.
1. Current “mental qualities” (in particular, gati and anusaya) are associated with the hadaya vatthu of the manomaya kaya.
2. Of course, gati and anusaya can and will change, even moment-to-moment. However, they change gradually. Even when the physical body dies, the manomaya kaya can enter another womb and start a new physical body, but with the “same” (but gradually changing) hadaya vatthu.
3. But when the “human bhava” ends, that manomaya kaya (hadaya vatthu and a set of pasada rupa) will die.
4. But kammic energy will create a new manomaya kaya (with a new hadaya vatthu and a set of pasada rupa) at that time. That is the cuti-patisandhi moment of “grasping a new bhava.”
5. That new hadaya vatthu WILL have the same anusaya (hidden defilements) but gati CAN BE very different from the those for the previous bhava.
– For example, if the transition is from a human bhava to an animal bhava, the new manomaya kaya will have “animal gati.” Note that even within a “dog bhava,” different dogs will have different “dog gati.” But as a set, all dogs have some common “dog gati” that are vastly inferior to “human gati.”
6. Therefore, while gati may change drastically, anusaya will remain the same after the cuti-patisandhi moment.
– We can say that dasa samyojana represent the anusaya of a lifestream. P.S. See “Dasa Samyōjana – Bonds in Rebirth Process.”
– Unless at least the Sotapanna Anugami stage is attained, ALL living beings will have the ten samyojana. A Sotapanna Anugami will attain the Sotapanna stage before the cuti-patisandhi moment.
7. Thus, by the time a Sotapanna gets to the cuti-patisandhi moment (to grasp a new bhava), three samyojana would have been removed from the anusaya.
– Thus, the new hadaya vatthu will have only SEVEN samyojana, i.e., less anusaya.
8. That means that lifestream CAN NOT grasp a bhava in an apaya. Those lost three samyojana are permanently lost.
– In other words, a Sotapanna can NEVER again have “dog gati” or the “gati of a hell-being,” for example.
– If that Sotapanna grasps a Deva bhava, they will have Deva gati. If a Brahma bhava grasped, then Brahma gati, etc.
Some familiarity with the posts on gandhabba may be needed: “Mental Body – Gandhabba” and “Gandhabba (Manomaya Kaya).”
Regarding the second part:
It s a good idea to write the comment/question using a word processor and then copy and paste, especially if it is a long comment.
– Also, note that a given comment SHOULD NOT be revised after an hour. It will not be revised. Some people had the original post deleted when they tried to revise it after an hour.