When changing a PGP passphrase, does it only affect the same key on the other machine?
If I change the password of my PGP key (I use it to sign Git commit) on Computer A, does it affect the same PGP key that I have on the Computer B?
Will I still be able to use the PGP key on Computer B with the old passphrase?
The passphrase of a GPG key only protects the specific storage of the key, i.e. it is basically a file encryption. Since the key in the computer B is only logically the same (i.e. same data) to the key on A but physically different (different file on different computer), the changes on the passphrase in A will not change anything on B.
Of course this will be different if you have some kind of file synchronization between these computers or if the key is stored on a shared storage like a network file system.