Let me go through what you are saying really quick and try to help. I'll assume you are logging in as a user with a standard set of access rights, you will always be prompted at first login to reset password unless the adminisrtator online
scripted a password for you with the value of false for forcepasswordreset.
However you are saying you are in the middle of a migration? If you are migrating mail then your account may not be ready to be accessed depending on the type of migration.
Do you have the admin login for the portal? You can reset by access
https://portal.microsoftonline.com and entering in your credentials there. Once on the landing page go to users on the left hand side, find your user account and reset the password there.
Depending on technical depth, you can also use the powershell to do this, by connecting to the powershell service for online administrators, and resetting the user account password there.