Per User Relay to From Old to New Postfix

[Asai]

Greetings,

Our organization is deploying a new upgraded Postfix server and we're 
looking for various options to migrate users from the old to the new.  
One possibility, we hope, is to do it on a per user basis rather than 
doing it all at once for a whole domain (which is also viable, but seems 
to result in more downtime for users until IT staff can make their 
rounds to their computers).  I have done some reading of documentation, 
but I'm not clear on whether using relay_recipient_maps is an option, or 
perhaps the transport_maps directive.

So, is this possible in Postfix, to relay email from the old to the new 
Postfix server on a per user basis, if so, what are some good options?

Thank you,
Asai