What then suprised me greatly was that her parrents had adopted the name Eichler.
It is vey likely indeed that those people who were your distant ancestors in West Prussia, had at one point decided to renounce their Polish heritage completely and after having chosen their new German one, they wanted to close this process symbolically by adopting a new German-sounding surname.
It is quite possible that those ancestors with the Polish-sounding surnames had ceased using Polish at home long before they decided to adopt their German name and felt themselves German rather than Polish/Kashubian. They may (or may not) have been aware that Polish was spoken by their grandparents or to a lesser extent or not at all by their parents. Anyway, their attachment to the Polish language may have been very week or none at all at the time of this surname changing.
The region of West Prussian ceased being Polish and became Prussian as a result of the First Partition of Poland in 1772.
Moreover, the father of Irmgard Eichler may have been a kind functionary or clerk in the Prussian administration which could additionally prompt him to change the name from "Dombrowski" into "Eichler".
Notice also that he decided to retain some link to the old surname by chossing "Eichler" rather than any other German name. "Eiche" means "oak tree" in German, so exactly the name of the same species of tree as found in his Polish surname "Dombrowski/Dąbrowski" which name derives its origin from "dąb" (oak tree) or "dąbrowa" (a group of oak trees or an oak wood). That way, the original roots of his family has not been completely erased, even though they were so wittingly hidden. It also means that the man knew the meaning of "Dombrowski" in Polish.
And last but not least, it is not that uncommon for some people to renounce as vigorously one's original language and ethnicity as even going to pondering upon a lobotomy of one's own brain (post #169):
https://polishforums.com/language/words-sound-funny-49893/6/
