Save an empty project from your customer’s v7.8. Open the .proj file in a text editor and examine the version headers. Edit the v7.9 .proj file to have the same headers and then attempt to import it. If it fails, examine the error logs to find what features of v7.9 you’ve used that won’t import. Edit under v7.9 to remove those elements. Repeat until it works. Then re-implement the stuff you had to delete.