The baro does not get changed during calibration. It's errors are just
plotted against the test chamber. So if it's 1000' off, then that is
applied to the flight in question.

For a mechanical barograph, a "golden" graph is created to be used in
measuring the altitudes. Over the years, this curve may change due to
aging of the bellows, etc.

Electronic barographs pretty much stay constant, but every two years,
one must make sure this is so.

Hope that helps.

-Tom