As far as I am aware, you would need to get in touch with Cosmote and ask them to swap the subscription over for you since Spotify don't have access to their systems.
If you have any issues, get in touch with the customer services team directly using the online contact form and they will be able to lend a hand.
If you get an automated email reply back directing you to the community or help pages, you need to reply directly to that email (even if its from no-reply) and one of the customer services agents will get back to you as soon as possible.
Peter
Spotify Community Mentor and Troubleshooter
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