Yes, it's supported to deploy the hybrid in forest A, and remove the hybrid configuration, then deploy the hybrid on forest B.
When do you want to change the primary SMTP address? Before or after deploying the hybrid in forest B?
If all mailboxes are move to O365 and the hybrid configuration is removed, you can change to use fabrikam.com from O365. If you still have mailboxes on on-premises Exchange, you can change UPN and primary SMTP address before moving to O365.
Additionally, you can check this for more information about adding a new domain to Microsoft365: Add a domain to Microsoft 365