You would only lost the link it the relationship is based on the document title. You will have to edit both the subdocuments and the documents if you linked by the title of the documents. This is not the suggested method to link your records, you should have a serial key field to link your documents.
Adding my voice to the previous comment. The primary key, or match field, should be a meaningless auto generated serial number. Your question illustrates the need for this very well. It's like linking on a name. Then a person gets married and her name changes. All links associated with the original name are then broken.