Exchange 2007 Migration

I'm looking at closing down out Exchange 2003 server which as been running for 5+ years now and really needs retiring. I have a nice shiny Dell PowerEdge 1950 attached to our SAN all ready to have Exchange 2007 installed.
My only question is this:

Is it OK to run all the domain prep commands and install 2007 +migrate all mailboxes as a test without affecting the origional 2003 Exchange i.e. get Exchange 2007 up and running locally (within the same domain) with all mail transferred to test out all the steps and processes first?
Without stuffing the domain and mailboxes of course :)

Exchange 2007 has a habit of taking over outbound message routing if you don't have a SMTP connector set up on your 2003 box which is something to watch out for. When testing I only had both running simultaneously for a short period of time so I am unsure of any other impact.

You may also want to have a read of this: Tips for a single-server upgrade from Exchange 2003 to Exchange 2007