Take a look at this post:
https://forums.autodesk.com/t5/revit-api-forum/reload-link-cad/m-p/6032592#M14312
You can actually reload a cadlink in Revit API 2018.
The categoryname of a cadlinktype is the same as the name of the importinstance: import instance name > find corresponding cadlinktype > cadlinktype.reload
Regards,
Raymond