I’ve never been able to get them off without damaging them.
Here’s what I do:
Get some pure acetone from the hardware store or Walmart (beauty products section). Holding the whistle with the head pointing down, using a glass eyedropper, drip two drops of acetone onto the joint where the whistlehead socket joins the tube. The socket will crack almost instantly.
Then remove the whistlehead and if it’s a brass tube, use some acetone on a tissue to remove the lacquer from the part of the tube that goes in the whistlehead socket.
Replace the whistlehead on the tube. The whistlehead will now fit correctly, with the crack slightly opened up by the tube. Take some sewing thread and wrap the socket end of the tube to completely cover the socket and reinforce the area over the crack. Paint the thread wrapping with clear nail polish to cement it to the socket and create a nice finish.
Now the whistlehead will be strong enough, and it will fit just right to allow comfortable adjustment.
(And, yes. I do this with every key of C and also key of F Generation I tweak.)
Best wishes,
Jerry