At work we want to migrate to Exchange online from our current setup of using rackspace hosted POP email. We have about 50 people planned for migration and want to minimize email downtime for them as much as possible. I have been reading Microsoft's provided documentation on migration but most of it is written for migration from a local exchange server. Does anyone have any experience in this migration process? I want to do some testing with my email account making sure it works but it seems like the tools only provide for an all or nothing on the migration.
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Additionally you can make use of the pst import functionality in Office 365. You can upload all PSTs to Office 365 and then import them to the newly created mailboxes. This way your users can start using their new mailboxes while in the background all their emails are imported. More details can be found here: https://technet.microsoft.com/en-us/library/ms.o365.cc.ingestionhelp.aspx
My suggestion will not cover the user migration process because it may vary from hoster to hoster (reading your question I suppose you are evaluating BPOS).
What you can do is:
1) configure client with a new profile (i.e. Exch) for exchange online
2) in exch-profile add a new data file (.pst) and select the file of the POP profile (you will have exchange mailbox and local-pop mailbox)
3) reopening Outlook let the user to use the POP profile (you can set it as default) until you enter in production with exchange online
When you will be in production with exchange online set as default the exchange profile (doing so the user will not open the wrong profile) and let the user move from local mailbox to exchange all the messages he wants/needs without downtime