Hello Dolgand,
It may be that your Cutlass Black was at a discounted price at the time.
For downgrades to ships/packages that are a priced less than your upgrade, you will need to contact RSI Support - https://support.robertsspaceindustries.com/hc/en-us/requests/new
I had the same issue downgrading a Polaris game package down to a Perseus.
Kind Regards
Zepheris