How is the validity of a private key in WIF format determined?

While trying to change the string of a previously acquired WIF private key, I rarely got a valid address.

Wallet Interchange Format (WIF) has a 4-byte checksum. This means that almost all changes will fail the checksum test, meaning that the changed string is not a valid WIF.

The whole purpose of WIF is to detect and prevent the use of such changes, i.e. to detect transcription errors.