I just did a similar upgrade last night from 18.04 to 22.04 LTS. No real problems.
First make sure all 18.04 packages are fully updated and that you have the latest MIAB version for 18.04 (I believe it was 0.57b)
Then follow these steps from @mouse
If you, during the final update of MIAB, hits an error about some python “blinker”, the do the following:
apt remove python3-blinker
sudo /usr/local/lib/mailinabox/env/bin/pip install --upgrade rtyaml "email_validator>=1.0.0" "exclusiveprocess" flask dnspython python-dateutil expiringdict gunicorn qrcode[pil] pyotp "idna>=2.0.0" "cryptography==37.0.2" psutil postfix-mta-sts-resolver b2sdk boto3
Then run the mailinabox installation again and it should all go smoothly