Updating Rev's of assemblies in jobs

Can anyone tell me why, when I go in and update a rev of an assembly in a
job, that if removes all the scrap rates that were put in when the job
details were originally put in? I first noticed this PROBLEM when I would
make a qty adjustment in a job - to take in account excess parts from a
previous job. Now I have noticed that I have the same PROBLEM when only the
rev letter!!??