Server decommission: Move mailboxes on same DAG

Hi Guys,

I'm doing server decommission for Exchange 2013. First, I need to move all mailboxes to the servers I'm going to keep. So, basically I have 14 server in a DAG and will decommission half of it. 

Scenario:

Same DAG

Servers 1,2,3,4,5,6,7 (To be decomm) 
Servers 8,9,10,11,12,13,14 (To be kept)

Server 1 Mailboxes (Databases) to Server 8 Mailboxes (Databases) etc..

With this scenario I just realized I'm just swapping the mailboxes back and forth to the databases copy on the servers. I have other DAGS.

Please help what will be the best way to handle such.


Thanks!
Uddan