Microsoft Online Services Team Blog has a write up on how to transition from BPOS to Office 365.
For any Software as a Service provider, it is critical to release updates and deliver changes to services in a way that balances continuous improvement with the flexibility to allow customers to control when releases and changes are delivered. Microsoft Online Services provides this balance by offering all existing BPOS customers up to twelve (12) months to transition from BPOS to Office 365. With that flexibility, you can make the transition to Office 365 when it makes sense for your business.
Further reading: Office 365
