I have performed some research and in fact it seems it was not very common, and if it was - being an unintentional result of mistakes or to make life simpler for officers.
In comments I've shown an example. One of main characters of All Quiet on the Western Front by E. Remarque (Am Westen nichts neues), being a Polish from Poznań (Posen), named Stanisław Kaczyński, was spelled in the book Stanislaus Katczinsky (compare German Wikipedia), however now I've changed my mind and think it is a spelling error of the author.
There is also another Pole in the book, Lewandowski, however there is nothing to change so it was more German-like.
The authors of Polish webpages concerning subjects like Kulturkampf, Hakata or germanisation do not say there was intentional changing Polish names to German ones (it was performed for Jewish names, but it's another story). Of course, a fact that something is not told about, does not mean it does not exist.
What is said above is rather concerning Posen.
This blog is titled "A word from a Silesia-woman about her tiny Homeland". The linked entry the author excerpts a book by priest Jan Nowak A chronicle of the city of Tarnowskie Góry and the land. The oldest Silesia events in Bytom-Tarnowskie Góry. A story about first Polish mining, written in 1927.
Then there goes a quote, however this dates 1743 (so earlier you expect):
I recall for example the surname Stefański; I've seen myself a signature with no errors, but on the very same page an annotation of a Prussian officer without any shame Stephainsky, so in one move five errors in a beautiful Polish name. This is not the end: later on in the document he'd changed the whole surname writing shortly Stephan.
So this made a German from a Pole. Some of surnames were changed unrecognizingly, eg. Szedoń was written by an officer Schädler, instead of Rajczyk - Reitzig and he without any consults baptised Poles as "native Germans".
Then the blog author mentions her own reminders:
I was once told by an old guy, that during "Hitler" times it was similar. People of Silesia were forced to change names, if only there was a shadow, a spark of Polish. It was also in his case. As a Silesian, he had a Silesian surname: Furgoł (...) They forced him to change to Flieger.
What might be also interesting for you: this is a Polish Wikipedia page Germanizacja na ziemiach polskich. There is a section Polscy działacze społeczni przeciwstawiający się germanizacji ("Polish social activists opposing the germanisation"). I have tried to open each of the person in Polish Wikipedia and then find out how it is spelled in German. This is an example of probably most notable person of the time: Dezydery Chłapowski, who is spelled correctly. For others you might need to follow cited sources. For Wojciech Korfanty it says he "was born Adalbert Korfanty" ("Adalbert" is German equivalent for "Wojciech", like German "Johannes" is equivalent for English "John").
Please also note that the examples I've found are dated outside the German Empire (before and after).