proxy dual server setup clarification

[Marc Roos]

I want to migrate mail users to a new environment. 

If I setup a new server next to the old server, and enable proxy on 
both. Is this then enough to migrate user by user with the passdb 
host=newsvr option to the new environment?

Eg. I do not need to configure a director setup? 
I only need to enable proxy for imap and lmtp on both servers? (messages 
on the oldsvr will be proxied to the newsvr?)