Is there an 'official' way to do this?
When I want to replace a format file and keep the old one I do this:
[calibre]Open the 'book' folder;
[filemgr]Create the 'data' folder if it doesn't exist;
[filemgr]Copy the file into the 'data' folder;
[filemgr]Close the 'book' folder;
[calibre]Remove the file;
If not, I suggest it could be in the Book details Formats XXXXX context menu, viz:
BR