Powershell for replacing primary email address and adding current primary to alias email addresses

We use a hybrid O365 environment.  On-premise AD, using Azure AD sync, with all O365 mailboxes.

We have about 120 mailboxes where we need to change their primary email domain to a new domain and ADD the existing primary email address as an alias, without deleting the existing aliases.

We would like to use powershell, importing from a .csv file, but cannot determine the powershell syntax needed to handle both the primary replacement, and the addition of the current primary to the proxyaddresses field, without wiping out the existing proxyaddresses list.

We were also considering running two separate powershells to handle this.  The first one would replace the existing primary with the new email address, but we read that when you do that, it automatically adds the existing primary as an alias, which is what we want, but deletes the existing aliases, which we definitely do not want.

Any help with a single script that would replace the primary, and ADD the existing primary as an alias would be appreciated.  If a single script can't do this, is there a way to replace the existing primary with a new email address that does not wipe out the existing proxyaddresses list during the process?  We would then run a second powershell to add an alias to the list.

Thank you - April