Migration was a process of transferring old accounts (created before February, 1st of 2017) from GFace services to the new servers.
As of February, 1st of 2018, migration is no longer available.
Please note that Support Service can no longer provide help with account migration either.
Keep in mind that migration was required only by accounts made before February, 1st of 2017. If you currently can access your account (either from the standalone version of My.com Game Center or from Steam), it means that there is no need for migration to be performed at all.
Thank you for your understanding.