We have a single exchange 2003 server that we're planning to migrate to exchange 2010. I understand that we would need to install them side by side and then transfer the mailboxes (amongst other things) over to the exchange 2010 server.

I understand that when the mailboxes are moved over mail would still be routed through the 2003 exchange to the 2010 exchange until we delete the routing.

What would happen with OWA, would users be able to use the OWA on the 2003 exchange to access the mailboxes on the 2010 exchange?

We're doing this migration in two stages and would not like to alter the firewall as we need minimum downtime.