Both the MCDX and MCD file types originate from Mathcad so it should be quite possible to re-save one another using the software. There doesn't seem to be any other way for this type of conversion.
Convert Mathcad Prime document file to MathCAD document file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert mcdx to mcd on Windows
A mathematical and engineering calculation software
Convert from mcdx file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.