Maintaining linkage between tables when downloading and/or uploading contacts database

Maintaining linkage between tables when downloading and/or uploading contacts database

Good day,
 
I have the need to download my contacts, perform some data manipulation, and then upload them once again.  How can I do this and maintain the links to the Vendors?  The download does not appear to include the linkage data that will reconnect the files when the upload occurs.  Or, when I upload will the uploading data merge with the already existing contacts? 
 
This is a real show stopper and if I can't do this we will have to move on.
 
WFL