When it comes to prehistoric belief systems, there is far more that is not known than is known. What seems to have been the case in Northern Europe is based on scant archaeological records with little or no documentation. It is not possible to conclude reliably that the available information indicates some dramatic, sudden shift.

It is possible to say that belief systems are in constant flux. This is particularly true when they are not tethered by some written canon, but even then, it is possible to demonstrate that religions based with written documents constantly adapt to meet current times and changing situations.

Neolithic and bronze age settlers of Northern Europe may have had a distinct set of belief systems and traditions that were different from those of the Germanic-speaking people who settled the north and then migrated elsewhere, providing various opportunities to have their traditions recorded by third-party observers and by themselves after adopting literacy (and typically after conversion, introducing its own set of problems).

There were likely differences between the traditions of these far-flung times and some of this may have represented some amount of in-migration of people, language, and/or religious traditions. Precisely how this process occurred is difficult to pin down and much debated without consensus. The process likely occurred differently in different places represented by the large amount of geography known today as Northern Europe.

Not much by way of answers here; in fact, this response represents a softening of the assertions embedded in your question, so we end up knowing less than when we started! Isn't that usually the way.