Decommissioning Hybrid Deployment

Question

Hi there,
We are currently running Exchange 2010 and an Exchange 2013 server deployed in a Hybrid set-up with all of our users now migrated to Office 365.
I plan to decommission both Exchange servers but after reading this article: http://blogs.technet.com/b/exchange/archive/2012/12/05/decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment.aspx it recommends keeping one Exchange server on-premise
or it will have 'adverse effects'.
What are these adverse effects and why should I need to keep hosting an on-premise server when it will not be doing anything?
Thanks
Ryan 

Answer 1

After speaking with O365 support they have advised that the 'adverse effect' is that you can no longer update certain mail attributes for AD users once the last server has been decommissioned.
For example you would need to use ADSI edit to add a proxy address, which is unsupported.
Decommissioning your Exchange 2010 servers in a Hybrid Deployment
http://blogs.technet.com/b/exchange/archive/2012/12/05/decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment.aspx
Mailbox Management
Organizations that have configured a hybrid deployment for mailbox management and hybrid feature support have also configured Office 365 Active Directory synchronization (DirSync) for user and identity
management. For organizations intending on keeping DirSync in place and continuing to manage user accounts from the on-premises organization, we recommend not removing the last Exchange 2010 server from the on-premises organization.
If the last Exchange server is removed, you cannot make changes to the mailbox object in Exchange Online because the source of authority is defined as on-premises. The
source of authority refers to the location where Active Directory directory service objects, such as users and groups, are mastered (an original source that defines copies of an object) in a hybrid deployment. If you
needed to edit most mailbox settings, you would have to be sure the Active Directory schema was extended on-premises and use unsupported tools such as Active Directory Service Interfaces Editor (ADSI Edit) for common administrative tasks. For example,
adding a proxy address or putting a mailbox on litigation hold when there isn’t an Exchange Management Console (EMC) or Exchange Management Shell (Shell) on-premises becomes difficult and these simple (and other more complex) tasks cannot be done in a supported
way.
Note: A hybrid deployment is not required in order to manage Exchange objects from an on-premises organization. You can effectively manage Exchange objects with an on-premises Exchange server even
if you do not have an organization relationship, Federation Trust, and third-party certificate in place. This Exchange server gives you a supported method for creating and managing your Exchange recipient objects. It is recommended to use Exchange Server 2010
for management tasks since this will give you the option to create objects such as remote mailboxes with the New-RemoteMailbox cmdlet. The server role needed is at least a Client Access Server (CAS) role, for management tools to work properly.