When you went back in time to prevent Hitler's entry into art school so that you wouldn't have to analyse his overrated aquarrells in art class anymore.
The joke is that a person named Hans Duchtelstein was the previous timelines "Hitler" and after being successful Hitler just stepped in because fate is inevitable.
Reverse: we time travel and kill Hitler, come to the present and Hans Duchtelstein takes Hitlers place but does the exact same thing.
Fun fact: Hitler's father was called Alois Schicklgruber until he was 40.
A pretty convoluted and sketchy turn of events surrounding a small inheritance lead to him being forced to change his last name to Hitler, 10 years before his son Adolf was born.
It's debatable whether Germans would have been as eager to shout "Heil Schicklgruber".
They would opt to "Heil Führer" or something. There are other ways to refer to someone than their given name but it's a fun fact indeed, thanks for sharing!