You should be able to decrypt the messages if you still have the old device to verify the new connection, or if you saved the encryption key that's setup when you first login to the account.

Appreciate it's not as simple as logging in, because that's the point of separating the encryption key from login creds in the first place. Even if your accounts credentials are compromised, previous messages aren't, and any future messages from the unverified device are flagged as suspicious.