Dears,

i am having exchange 2007 installed on 4 servers , 2 mailbox and 2 hubcas servers and will migrate 2 exchange 2013 with highly available solution with 2 mailbox DAG and 2 cas servers in a network load balance.

i want to know if i could begin the migration with only one mailbox server and one cas server and later on add the other mailbox and other cas server

yes, you can do that.

1. First install and configure one 2013 CAS server - for e.g. CAS01

2. Install MBX01, create mailbox databases and move some mailboxes from 2007 and test

3. After successful testing install second CAS - CAS02

4. Install MBX02 and create mailbox databases

5. Configure DAG, add mailbox servers and add database copies

6. Configure NLB/HLB

7. start pre decommission activities like cut over to 2013

8. uninstall 2007