What do I need to do before migrating Exchange Server to Office 365 ?

Hi,

I have Exchange Server 2010 SP3 total number of active mailbox 2500 and total Mailbox database size is 8 TB+ that I will migrate to Office 365 this end of year.

My clients are using Outlook 2010 & 2013 spread across multiple geographical region site offices (Europe and Asia Pacific).

Note:

• I also have emails older than 1 year archived by Symantec Enterprise Vault 10 with approximate size is 7.7 TB
• Single AD domain forest.

What do I need to do before migrating to Office 365 to ensure the archived email still accessible by Enterprise Vault and EV can still archive email in house / on-premise ?

Any thoughts and comments would be greatly appreciated.

Thanks in advance.