Fix a broken content type in a document library

We have a content type which is used by a number of document libraries.  In one , and only one library, the content type has got corrupted.  Going into Library Settings >> More Library Settings shows that it contains the correct column heading names , but all the more specialist data types (Managed Metadata, Date and Time etc ) have regressed to being a Single line of text. 

If I am creating a new document library , it behaves itself 

I cant delete the content type from the list & reapply it because its in use 

Can any one recommend a way to make sure that whatever version of the content type is stored in the document library is refreshed with the appropriate settings from the content type.  Thanks in advance