One of the most popular “workarounds” for not having backup&restore data other than the deleted items retention period when you migrate to Exchange Online, is to configure Exchange Hosted Archive for all of some mailboxes within the Organization.
Microsoft recently published technical documentation that might help doing the desired configuration: Microsoft Exchange Online Configuration Guide to work with Exchange Hosted Archive.
This document provides the necessary steps to properly configure Microsoft ® Exchange Online Standard to work with Hosted Archive service of Exchange Hosted Services.
Exchange Hosted Services Archive (EHA) is embedded in the Microsoft Exchanged Hosted Services (EHS) network and provides a repository that stores e-mail and instant messages. EHA can be used by your organization to assist in managing increasingly complex retention, compliance, and regulatory requirements. This document provides the necessary steps to properly configure Microsoft ® Exchange Online Standard to work with Hosted Archive service of Exchange Hosted Services.
Please refer to the following information when you configure Exchange Online to work with EHA.