iMessage has end to end encryption and supports multiple devices. Each device uses its own keypair and when a message is sent, it is sent encrypted for each of the recipient's devices.
If it's because Apple tells me so then I think can see that the end to end encryption is broken and interception is possible.
Are old messages re-encrypted with the new key?
This would be even worse.
It would still be possible to implement this in a secure way (including out-of-band key verification) by having the "original" key (the one you've compared out-of-band) sign keys for new devices and have the server deliver that signature (which the sender can then verify). I don't know if there are any implementations of this.
The thing which is really sad is that there are no end to end encrypted group chats.
In iMessage specifically, or in general? WhatsApp/Signal do use E2E-crypto for group chats.
e2e is in beta still, but they have key list management/verification in place already.