Timing moving from Exchange 2007 to Exchange 2010

I am in the process of upgrading from Exchange 2007 to Exchange 2010 and have questions.. both systems are running in parallel.

When should you change the DNS entry for autodiscover?
When should you change the clustered systems and destination server settings on a Barracuda Spam Firewall?

I am especially concerned about the autodiscover and ActiveSync settings so that the new Exchange 2010 users will have access immediately after the move.

Donna K. Appleget