Just called Amex and got a bit of a run around. While waiting for their call back, I struck gold - I now have two Bonvoy accounts - and if I log in with the SPG number (as shown on the Amex benefits page) I'm still Gold.
Logged into my account number that was linked on the Amex benefits, and yes I'm still gold if I login with my SPG number. It seems somehow I have 2 accounts, 1 silver and the other gold. Rather confusing.