Exchange 2003 to 2010 install question

I am in the process of deploying Exchange 2010 in an Exchange 2003 environment. I have two servers that wil hold the CAS/HUB and Mailbox storage. My question may be pretty simple but can I install the CAS servers, configure the CAS array without configuring 2003 coexistence? Can I configure coexistence at a later time?

Thanks!