Our current on premise Exchange infrastructure is Exchange 2010 SP3. To deploy a hybrid infrastructure with Office 365 (version 15), will I need an Exchange 2013 CAS server for federated free\busy and mailbox migration? Or can I leave everything as is Exchange 2010 SP3 and proceed with a hybrid configuration.
Another question, Id like to know is that our current O365 tenancy is version 15. Would it be possible to request a O365 tenancy that is at version 14 in place of what we currently have?