Modifying the existing journal really sounds like the wrong solution. Just "journalctl --rotate" the file and throw out the one with accidental PII. Journal files are not great for long-term storage or search. You can export the old file and filter out manually if you really want to preserve that one https://www.freedesktop.org/wiki/Software/systemd/export/
In what situations is it a harder problem than this?
> I once typed an SSH password in the username field, and the only way to erase that was to erase all the logs. So this has some significant downsides.
I hope this was a personal system. Changing logs in this manner would have almost certainly led to your dismissal anywhere I ever worked. This anecdote just re-enforces the need for Forward Secure Sealing.
Per https://systemd.io/JOURNAL_FILE_FORMAT/, the benefits of the binary format are:
The systemd journal stores log data in a binary format with several features:
Fully indexed by all fields
Can store binary data, up to 2^64-1 in size
Seekable
Primarily append-based, hence robust to corruption
Support for in-line compression
Support for in-line Forward Secure Sealing
As a user system-administrator, I see the cryptographic checksum as a benefit of being able to show tampering evidence of on-system log files.