These are my top 10 tips and learnings made by leading Office 365 migration projects:

1. Plan your User Identities

Identities are everything in Office 365. Spend more time on planning the identities than on any other service. While for SharePoint/Exchange/Skype/etc. you will be able to do some changes during the migration, if you need to change something for the identities or authentication it can become a nightmare as it affects everything in Office 365.

e.g. you cannot change the sourceAnchor attribute (it is immutable during the lifetime of an object)

2. Use idFix to fix your Active Directory before uploading the users to the cloud.

IdFix is used to perform discovery and remediation of identity objects and their attributes in an on-premises Active Directory environment in preparation for migration to Azure Active Directory.

https://www.microsoft.com/en-us/download/details.aspx?id=36832

3. Create a proof of concept (Poc)

Test the complete migration procedure in a lab or test environment. You will be able to use the environment also for additional testing or issues that might encounter post migration.

4. Evaluate 3rd party tools and ask for trial licenses

Sometimes the Microsoft tools are not enough and you need to go for third party tools. Contact the sales persons and ask for a demo. Many times the sales persons are also the ones that can give you a free trial to evaluate the tool in your test environment.

5. Create a migration runbook for sysadmins

IT administrators from other locations should be able to perform a complete migration by following the steps in the runbook. It doesn’t need to have a specific format, but it needs to be very well documented so every newbie would be able to follow it.

6. Migrate some test users and the IT administrators first

Do not start the velocity migration before getting the confirmation from the local departments that all processes and tools work as expected.

7. Office 365 throttling

Office 365 uses various throttling mechanisms to help ensure security and service availability. You might fight out during the migration that data is being transferred to the cloud at a low speed, and not because of your local network. There are various methods to overcome those throttling policies (e.g. for Exchange you can ask Microsoft support to „relax” the throttling for your tenant during the migration).

8. Change Management

Take care of your users. Communicate all change to users and train them before the migration itself. Remember that a successful project is being measured through happy users.

9. Have a disaster recovery and backup plan

Mistakes can happen. But if they happen, you need to solve them fast.

10. Use the FasTrack Center Benefits

If you have 500 or more Office 365 licenses per tenant, you qualify for the FastTrack center migration services. Do not hope that FastTrack will do the migration for you 😊

Their benefits are described here: https://docs.microsoft.com/en-us/fasttrack/fasttrack-benefit-for-office-365