Both .cap and capx files are used by Capella music notation software, they just that represent various version of the same format. It seems that you can simply convert (re-save) one to another using the original software.
Convert Capella music sheet to Capella music sheet data file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert capx to cap on Windows
Capella is music scores writing software for Microsoft Windows.
Convert from capx file
to cap file
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.