In our willingness to believe any evil of Trump, some have claimed his original name was "Drumpf". This isn't true, this isn't how the German language works. Trump has the power to short-circuit critical thinking in both his supporters and his enemies. The "Drumpf" meme is just one example.
There was no official pronunciation or spelling of German words/names until after Trump's grandfather was born. As this The Guardian article describes, in the city ("Kallstadt") where Trump's grandfather was born, you'll see many different spellings of the family name in the church's records. like "Drumb, Tromb, Tromp, Trum, Trumpff, Dromb" and Trump. A person might spell their name different ways on different documents, and the names of children might be spelled different than their parent's. It makes German genealogy tough sometimes.
During that time, different areas of German had different dialects that were as far apart as Dutch and German are today. Indeed, these dialects persist. Germans who grow up outside of cities often learn their own local dialect and standard German as two different languages. Everyone understands standard German, but many villagers cannot speak it. They often live their entire lives within a hundred kilometers of where they grew up because if they go too far away, people can no longer understand them.
The various German dialects, sub-dialects, and accents often had consistent language shifts, where the same sound is pronounced differently across many words. For example, words that in English have a 'p' will in German have 'pf" instead, like the word penny becoming Pfennig, or pepper becoming Pfeffer.
Kallstadt is located in the Pfalz region of Germany, or as they pronounce it in the local dialect, Palz. You see what I'm getting at, what is 'pf' in German is 'p' (like English) in the local dialect. Thus, you'd say "Trump" if you were speak Pfalz dialect, or "Trumpf" if you were speaking standard German.
It's like the word for stocking, which in standard German is Strumpf. In documents written around that time in the Pfalz region, you'd find spellings like Strump, Strumpf, Strumpff, Strimp, and Stromp. Both the vowels and the last consonant would change (according to a Pfalz dictionary I found online).
Friederich Trump was born in 1869, in a time when Germany was split into numerous smaller countries. The German Empire that unified Germany was created in 1871. The counsel to standardize the language and spellings was 1876. Friederich emigrated to America in 1885. In other words, his birth predates the era in which they would've standardized the spelling of names.
From the records we have, "Trump" was on his baptism record, and "Trump" is how he spelled his name in America, but "Trumpf", with an 'f' was on his immigration form. That's perfectly reasonable. The immigration officer was probably a German speaker, who asked his name, and spelled it according to his version of German, with an 'f'.
This idea of an official spelling/pronunciation of a name is a modern invention, with the invention of the modern "state" and "government officials". It didn't exist back when Friederich was born. His only birth record is actually his baptismal record at the local church.
Thus, Trump's name is spelled "Trump". It was never officially spelled any other way in the past. It was never "changed". Sure, you'll see church documents and stuff with different spellings, but just how all words and names were handled back then. Insisting that he's "Drumpf" is ignorant -- it's not now the German language works.
Update: Somebody named Gwenda Blair wrote book on Trump's family, which claims the name comes from Hanns Drumpf, who settled in Kallstadt in 1608. But they can't connect the dots. That's because right after, the 30 years war happened. It's a famous event in Germany because it burnt most of the church records. Most all German family trees can be traced back to the 30 years war -- but no further.
It's probable they were related. It's possible that Hanns was even an ancestor of Trump who at one time spelled his name "Drumpf". But that's still not the official spelling, because that's not how German worked at that time.
Update: According to Snopes, it's true that "Donald Trump's ancestors changed their surname from Drumpf to Trump". Snopes is wrong, because they are morons. The correct answer is "there's no record of a name change". The fact that difference sources make conflicting claims should've been proof enough for Snopes that there's no evidence to support the claim.
BTW, my great-grandmother is "Pennsylvania Dutch", most of who came from that same region. I may be distantly related to Trump.
Also BTW, isn't weird that we are talking about his grandfather born 150 years ago? His grandfather was 36 when his son was born, and his father was 41 when Trump was born. Three generations back, and we are already in a pre-historical era -- that is to say, the era where we had writing, but not standardized spelling.