Tobias wrote: “..But we all have heard about people who saw the new distination already days before the death. ..Will the new bhava be confirmed only in the last moments of the old life?”
Yes. That ārammana (thought object) starts to come to the mind even days before cuti-patisandhi moment in the last citta vithi.
P.S. This happens to reinforce that ārammana in the mind.
Suppose it is a human who is about to grasp a new lower bhava (in the apāyās). If the person has removed “apāyagāmi gathi”, that ārammana will not take hold. If it does not take hold, then the “next strong vipāka in line” will come to the mind. That will go on until an ārammana for a “good bhava” (compatible with the Sotapanna stage, for example) comes to the mind. Then it will not be rejected by the mind, and it will stay there until the last citta vithi, where it will be grasped.