point is you have to ask for it, the Fed's aren't just going to do it because you got a new rating.
My certificate # was changed to a non SS based # in April 2006. I did not ask. Heck, I was not even aware of the change until late 2007. How often do you look @ your certificate #? Well, I don't often do it. I checked the address, the spelling of the name, and the ratings and endorsements, not the certificate #.
The point is you should ask for it, but you don't always have to.
When I got my CFI back in 2003(?), it was not even a choice at the GRR FSDO. He said this is your new cert number, and I asked if I could keep the old one. He responded no I couldn't but it's for you own security.