why dont you exports the logs to a central repository and later you can refer to those
there are various export utilities available depending upon the trype of job. for more details check blcli help.
This is by design. It's related to the data model and the relationship between objects.
If you are modifying an existing Job that already has JobRuns, I'd suggest you copy the BatchJob and do the modifications on the copy. It follows best practices of content management: if an object is altered, it should have a different version and therefore be a separate copy of the object.