From the Unicorn FAQ:
"And my big Racks?
They stay big. But you can not enlarge them anymore and you can not create new ones if this would hit the limits."
This was my understanding when I paid for a Unicorn account: that I would not be able to create or enlarge racks that were over the size limit if the I allowed the Unicorn account to expire. I did expect that I'd still be able to change modules on the existing large rack, however--which isn't the case.
The FAQ should be updated to reflect this.