The key is signed using digest algorithm 2 (aka SHA1):
$ gpg --list-packets PlexSign.key
[...]
# off=539 ctb=89 tag=2 hlen=3 plen=567
:signature packet: algo 1, keyid 97203C7B3ADCA79D
version 4, created 1427058047, md5len 0, sigclass 0x13
digest algo 2, begin of digest 90 2f
^^^^^^^^^^^^^
[...]
The key itself can be updated with new signatures using gpg --edit-key --digest-algo SHA512.
One would propably also update preferences in this step, e.g. setpref SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed (or whatever the desired preference is), if it’s not up to date.
Second part is the RPM package itself: see Legacy SHA1 signatures on RPM packages
With proper key preferences and no overrides the signing process should be fine right away. Otherwise the digest algorithm can be configured in the .rpmmacros or the command itself (%__gpg_sign_cmd %{__gpg} gpg [...] --digest-algo sha256 [...])
See e.g. RPM file format changes to support SHA-256 - Fedora Project Wiki
