I have exactly the same R1818 bill issue. I did upgrade shortly after my 2020 36 month contract expired, and after much pain was told I already owe the R1818 since it's the cost of the first month of the now month-to-month contract.
Even after upgrading, the full R1818 + R299 (for new package) is due.
To add insult to injury, since I upgraded last week on Tuesday (to try to prevent the massive bill), I now can't take any BF deal since the new month-to-month has to be active for at least 2 months before being able to switch to a contract. I thus need to take out a brand new additional contract if I want a BF deal, which will result in a total bill of R1818 (expired contact's first month to month) + R299 (current month to month) + R299 (new month to month BF deal).
I've pretty much given up, and om probably just going to ignore MTN BF, and look at other service provider.