You can change UPN on guest users using PowerShell. You can even drop the "#EXT#"-part, and use any verified domain in the guest tenant, not only the initial onmicrosoft address.
One problem with this, is that the visible username for the actual guest user when logging into Azure for instance is not changed. It remains the email address used to invite the user initially. Even though the SMTP address or UPN used for inviting is removed from both the source and the guest tenant, this is still shown in the username.

Request: Update displayed username for a B2B guest user if UPN is changed.