Is that on the cheque card only? Or credit as well?
My logic would be that if your credit card stays the same then there shouldnt be an issue.
Things were a real mess for me. I tried to convert to gold fusion from gold cheque + credit card. I stopped spending on my CC and their delay with payments clearing meant I had to wait for them before I could cancel it properly. In the meantime, they took it upon themselves to upgrade me to private fusion, and from then on it was a fight with lots of redirects to get back to gold. I lost out on a lot of ebucks, but once I had my gold fusion card, things were back to normal. So if you can somehow find the right person who knows what they are doing when you do the downgrade, perhaps you will be able to salvage the month - especially if you do what @Gtx Gaming suggested.
Every time they **** up royally, someone at FNB always asks "did you open your initial account online?" followed by an "ah - that explains it". It is a royal pain doing admin with them.