change first name and primary email address

Hi all, I have been tasked with changing the first name & primary smtp address of approximately 300 mailboxes, while retaining the legacy smtp address. There are multiple shared mailboxes with the same first name (321654.sales, HR, support etc)

current first name: 321654 to New name: Madrid

current first name 987654 to New name: Vienna

current SMTP: 321654.sales@domain.com to madrid.here@domain.com

current SMTP: 987654.sales@domain.com to vienna.here@domain.com

Etc

I have a csv file with the headers for "old" & "new" name but trying add the address piece to the powershell command

some pointers would be great while I scan the web :) and learn all sorts of other stuff!

thanks in advance