_Nothing_ history-relevant can be changed via manipulation of the Fossil db. In terms of db records, as opposed to space, the db is about 80-90% a transient cache of data which is generated from the remaining (100% immutable) data. Any changes you make to that transient data will be lost the next time that cache is discarded and rebuilt. A longer explanation can be found at:

https://fossil-scm.org/home/doc/trunk/www/fossil-is-not-rela...