That works for a while, but then Citi stops accepting payments to a closed card and reverses the attempts. I don't know how long before that happens--it could be years--but be careful using this method.
I'm aware it's a possibility. Cap1 did this within 2 months of shutdown. All that happened was CFP said I had to go back to WM and pick up a refund. At that point it's just a minor hassle and you can stop doing it.