I replaced the needle threader on my Pfaff once. It was easy. The entire unit just came out, like in your diagram above. And the new unit slides up in. The only thing I did wrong is the new unit isn't in the exact spot, so I have to use the handwheel to move the needle down about a sixteenth of an inch for the threader to pop into the hole of the needle. I remember the instructions said to make sure the unit is pushed all the way up inside. It seems like I pushed mine in too far, but that doesn't seem possible. Good luck, you can do it!