No direct method but I don't see why you'd need to reconvert.
Assuming you have 2 non-empty libraries A and B and you wish to copy the icon rules from A to B:
1. Create new library C, copy from current library A structure only. This will create an empty library C with the same icon rules as library A.
2. Switch to library B, select all books and copy to library C. This will copy all metadata (assuming all custom columns match) and all formats for all book entries from library B to library C.
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