Active Directory Synchronization in Office 365 differed from BPOS in a major way in that once it was activated, you could not turn it off – until now… (trying to make the topic as exciting as possible)
Keep in mind that if you choose to deactivate AD Synchronization in Office 365 it can take up to 72 hours to complete, during which time no synchronization from the existing environment would occur (obviously) but also you cannot immediately configure synchronization to another environment until this is completed.
While waiting for DirSync to finish for the first time at a client site, we received the following email (partially edited for confidentiality):
Synchronization errors for more information about the errors listed in this
The Directory Synchronization batch run was
completed on Tuesday, 03 January 2012 23:54:02 GMT for tenant <client>, Inc..
The following errors occurred during synchronization:
Synchronization has been stopped. The company has
exceeded the number of objects that can be synchronized. Contact Technical
Support and ask for an increase in your company’s quota..
Tracking ID: cb441337-5739-4d70-9b2e-4f10d87598a0
Directory Synchronization has a 20,000 object limit (or 10,000 for tenants created before Oct. 5, 2011); that may seem like a lot, but remember you do not get to control with the application synchronizes. DirSync will grab every user, contact and group. Luckily the way around this is simply to request a quota increase from the MSO support team. Keep this in mind during design sessions and it can save you a time-consuming hurdle during implementation.
More information on identifying and resolving the problem can be found here: http://support.microsoft.com/kb/2459803