Security Token for my store was changed. Why is it so?

It seems that Security Token of your account and Security Token of Connection Bridge downloaded to your website doesn't match each other. It could happen if Bridge was downloaded from another account that is used for your migration. To resolve this issue do the following:

1. Click ‘Continue Migration’ button on “My Migrations” page.
2. Get to the 1st migration step and download the Connection Bridge one more time. Unzip it and paste to the store's root directory.
3. After re-uploading the Bridge, proceed with your data transfer.