A heads up for everyone using OneDrive for Business sync client with tenants of less than 250 users.
Beginning in May 2016, Office 365 customers with fewer than 250 Office 365 licenses will be required to use the OneDrive for Business Next Generation Sync Client to sync OneDrive for Business files. This requirement will be rolled out between May 2016 and July 31, 2016 and will not apply to on-premises customers or customers with more than 250 Office 365 licenses.
I wrote an article about the different OneDrive Sync clients a while back that may also help answer some questions:
Basically, this will ensure all Office 365 installations with <250 users will be using the NextGen sync client.