y not said: There is an enduring ‘X’ but it is ever-changing, and that up until Parinibbana.
I think it is not like that. I could be wrong of course. When we think of an enduring ‘X’ which is still ever-changing, we risk conceptualising the self-identity as having a permanent core with only its outer aspects or manifestations changing.
It seems to me that we are a beginningless stream of kammic energy, fuelled by ignorance (avijja), craving (tanha) and grasping (upadana). Through patticca samuppada, avijja paccaya sankhara, vinnana, vedana, rupa and maybe sanna (which could be between phassa and vedana). Thus we are made up of the five skhandhas, which is further fuelled by the food we take in (ahara). Even the core (5 skhandhas) is ever-changing. Everything is in flux. We seem to conceptualise a united, fixed core because we somehow link this continuous chain of events in our memories to a single entity.
However, there is probably no need to worry about that. It might become clear once the arahant magga is reached. I could be wrong of course. It seems to be too advanced to grasp for someone who is not yet a sotapanna. I am only thinking about this intellectually of course, and have not verified it through meditation.