I tried the same thing and was unable to remove the head of an astrtomech. I wanted to see what the dark red head from the V-wing wouold look like on the dark red "body" of the droid from Ahsoka's Star Fighter. I liked the look better and bought the keychain version of that droid. Easily got out the keychain, but couldn't get the head off. I ended up compromising and put the the key chain version with the Star Fighter, placing the dark red head and body together for display, and placed the original white/green head with the V-wing. It made the astromech head actually stand out with the V-wing, which was a pleasant surprise.
Sorry I'm not much help with your problem, but I would be interested to know if anyone successfully managed to seperate the pieces for the astromech. The old key chains, I had a Darth Vader one from around 2002 or 2003, once the pin came out the helmet came off and revealed that there still was a printed Vader face underneath. These days I think they are glued, so it's probably impossible. Though I would like to be proven wrong.
"Never hold discussions with the monkey when the organ grinder is in the room." -Winston Churchill