The ACME client creates and handles everything about your “account” that is required when talking to the Let’s Encrypt servers that issue the certificate.
Normally with MIAB upgrades this isn’t an issue, because you’re running off the same original install (and LE “account”), just upgrading. With the jump from .30 to .40, you have to do a new install onto 18.04. During the install, LE creates a new account to handle the certificates, but your old account is also restored when you go through the backup restore process.
This should be a one time thing.