In GnuPG I have a master key that doesn't expire, with separate subkeys with one year expiration time for encryption, authentication, and signing. If I update the expiration dates of my subkeys, will I need to make new backup copies of my private master key?
The expiration date of subkeys is stored in a special kind of signature issued by the primary master key on the subkey. With other words, if you change the expiration date, no private keys are changed at all. If you can restore the public key later (for example, fetching it from the key server network), you're fine.