Microsoft is starting to divulge information on how a current Office 365 Tenant that is based on the 2010 Product sets (Wave 14) will be upgraded to the new Office 365 services with the 2013 products (Wave 15). The Office 365 Service Upgrade Center on the Community site is live, http://community.office365.com/en-us/wikis/office_365_service_updates/office-365-service-upgrade-center-for-enterprise.aspx. This site talks about new in the Service Upgrade, what the Upgrade experience will be like and has other resource links for the Service Upgrade.
Overall Microsoft has designed the Upgrade process to be as seamless as possible to the customers IT department as well as the end users. For the end users who use the Outlook client and access their mailbox on a mobile device the upgrade experience with be almost completely transparent. The only notification a user might experience is if they were accessing the Outlook client when the upgrade completed, is a message that they need to restart the Outlook client. Otherwise their Outlook and Phone will need no other configuration to continue to access the mailbox after the upgrade. For users that utilize the web portal and OWA, they will be prompted to re-login if they are accessing both when the upgrade completes and then with the next login will experience the new portal and OWA for Wave 15.
For the IT admins Microsoft is allow the company to Pilot up to 100 mailboxes to the Wave 15 version while the remaining users would still be on the Wave 14 version. This allows for trial of the upgrade experience as well as verify the end user experience with a select group of IT friendly users to prepare for the upgrade for the larger user group. Microsoft will be scheduling Upgrades for customers after Wave 15 is Generally Available (GA) and will give a 30 day lead time for a company to upgrade. This will be a rolling upgrade and I do not mean to state that all Wave 14 tenants will be scheduled for upgrade within 30 days of GA. This window of upgrade can be postponed by the IT admin for a minimum of two months. The caveat with this is you can only postpone once.
Speaking of the Office 365 Wave 15 Services, here is a link to the Beta Service Descriptions http://technet.microsoft.com/en-us/library/jj819284.aspx