Please see a list of below actions you’ll need to take pre-tenant migration;
Emails – All email data will be transferred across so no actions will be required here.
Calendars – All “recurring” calendar invites where you are the organiser will need the end date to be set before the migration and to be re-made when the tenant migration has completed. I would suggest creating a list prior to the migration ready to re-create those invites after the migration.
OneDrive – All OneDrive data will be transferred across. All pre-existing shared documents will need to be re-shared with the internal or external contact after the migration.
Teams – All teams chats are to be wiped so you will lose the history of conversations, please review and save any documents you’ve shared within teams’ chats.
Mobile Device – Please follow this guide to back up your data before the IT migration such as contacts, texts and photos if you have a company mobile device – Mobile Devices (sharepoint.com) /