While I was at it, I thought I might as well blog about it. In the situation you need to dismantle an Exchange 2010 DAG member, proceed as follows:

1. Start up Exchange Management Console (ofcourse you can do this from the Exchange Management Shell as well,  but in this example I’ll use EMC);
2. Go to Organization Configuration > Mailbox > Database Management;
3. Select the database where ” Mounted on Server” reads the server you’re decommissioning;
4. Select Move Active Mailbox Database;
5. Select the Mailbox server to host the  mailbox database copy and select Move;
6. When the move has finished, select the database copy hosted on the server you want to decommission in the lower pane. There, select Remove.
7. When finished you’re greeted with a warning message. That is because the copy has been removed, but the files are still present. Remove them manually when required;
8. Next, we need to remove the server from the DAG. Select tab Database Availability Groups;
9. Select the DAG the server is a member of and select Manage Database Availability Group Membership;
10. Select the server and click the red cross to remove it from the list. Click Manage to proceed with the actual removal;
11. When finished the mailbox server is no longer member of the DAG.

You can then proceed with uninstalling Exchange. Note that you can only remove DAG members of a healthy DAG.