Why exactly do negatives entities inevitably need to switch polarities?

[Aion]

As far as I understand you would only "lose a density" if you did it very shortly after 'graduating'. So if an entity just moved in to fourth-density negative, somehow suddenly decides it changes its mind I don't think it can go "further" in to fourth density to do that work but would have to do a great deal of third density work to make that switch because that's the only option really.

Alternatively, it's possible that such an entity might end up stuck on the 'lower fourth' levels and have to spend a great deal of time there in order to move on. In which case incarnation may actually be the 'faster' option. I think this scenario is preeetty unlikely or uncommon because I think the amount of effort put in to reach fourth density on either path is such a commitment that it would be kind of strange to just scrap that work, but I'm sure there are occurrences.