Update. I went to my local CostCo to downgrade my membership to the "normal" level... speaking to the friendly staff member, I asked about options for those with the Visa CostCo card and not longer having a membership at all. She mentioned that while the normal procedure is for the credit card account to be closed, that she does know of instances where Citibank can make "alternate" arrangements.... and she strongly suggested I call them to inquire about the possibilities.

I will take her up on this advice and see how it goes.... will come back and report my experiences.