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

Here is an informative blog from technet team that explains all the steps in depth by covering all the required aspects : http://blogs.technet.com/b/meamcs/archive/2013/07/25/part-1-step-by-step-exchange-2007-to-2013-migration.aspx

Moreover, if you wish to accomplish this task automatically, you may also have a look on this automated solution (http://www.exchangemigrationtool.com/) that could be a good alternative approach and get this job done in an easier way.