I read this entire thread start to finish over the past couple of weeks but I still couldn't believe the drama I had today detaching a certificate, converting it and reattaching it to a new reservation. Over 2.5 hours. The first representative from Mexico City didn't know what she was doing and was awful, she deleted the entire reservation and certificate and hung up when she couldn't figure out what to do next. Thankfully when I called back I got someone in Nebraska who was really helpful and able to track down what the other rep did and fix it. I couldn't get it converted to a Cat 5 but attached it to a Cat 4 hotel for now (since I was on the phone so long anyway) and will try that part again another day.
If I hadn't read this thread I would have been sunk so thank you for all your posts. The key is really detaching it before cancelling the reservation if you already have it attached. The rep didn't listen to me and deleted the reservation/certificate and because it was so old it didn't redeposit into the account automatically. Done now but crazy amount of work!