That’s what I want to know, but I would still need to get everyone to stop following the old npub and follow the new one, and that’s a lot to ask.
You can keep using your current npub and delegate down to it.
For example, the npub I'm using right now does double duty as the signer key for a cold storage identity. The profile page of that identity is here:
https://www.inkan.cc/users/npub10ukg94kvrvk4qqr3482zdekfsuaw2x56wa899mnpkxqwfxl6dlkqrux0x9
People who are currently following the signer key can continue to do so for as long as they like, although I'd hope that they'll eventually get around to directly following the identity key (since the identity key will survive if / when the signer key gets compromised).
For example, the npub I'm using right now does double duty as the signer key for a cold storage identity. The profile page of that identity is here:
https://www.inkan.cc/users/npub10ukg94kvrvk4qqr3482zdekfsuaw2x56wa899mnpkxqwfxl6dlkqrux0x9
People who are currently following the signer key can continue to do so for as long as they like, although I'd hope that they'll eventually get around to directly following the identity key (since the identity key will survive if / when the signer key gets compromised).