The Whole Process
The playbook for a successful migration to SharePoint Online usually goes like this:
1. Plan communications and support for the business
2. Inventory your content and customisations
3. Prepare to clean up any content and customisations you don’t need
4. Select a migration tool – preferably one that supports delta or incremental top-ups
5. Plan the migration including any steps to redevelop and restyle
6. Remember to coordinate your migration plan with the business to minimise impact
7. Perform a test migration with a representative subset of content and customisations
8. Test it works!
9. Perform the full migration
10. Test it works again!
But often, step 8 onwards is more like this:
8. Break your migration plan into manageable chunks
9. Establish your to-be SharePoint Online structure
10. Migrate, then test, one chunk of content at a time, making each one live as you go
11. Repeat until complete
When all this is done, keep your old SharePoint Server environment around in read only mode until you are happy to finally decommission it.