I presume that the fan you are working on is from a W124. if so, i have managed to get the plastic fan blade off of my genuine AEG fan which was irreparable due to corrosion... so i decided to tear into and see how its is put together. i managed to salvage some good parts from it and then installed them into the other fan which is now fully functional.

i removed the black sticker on top of the fan, and properly wedged the fan blade between two blocks of wood, and then used a hammer and a steel pin punch to knock the end of the electric motor shaft out of the plastic fan blade, it took some persistence but it eventually came off.

the fan is pressed on to electric motor shaft, NOT threaded on. the end of the electric motor shaft had some rust on it which was probably why i had a tough time getting it off, so you might want to try some wd-40 or something similar.

good luck
tim