As much as I have seen, if you tamper with the files of a .pkpass, Apple Wallet will refuse to open you pass since it "sees" that the checksum of files are different. Although the manifest should do this for you, the signature file also includes the original manifest of the .pkpass. Thus, even if you update the checksums of your files in the manifest file, Apple should still see that the pass has been modified.

I saw that when you adapt the checksums files, Google Wallet will open the altered .pkpass but Apple Wallet won't.